fix(search): restore unquoted multi-word date keywords via pre-parse quoting

"added:previous month" returned HTTP 400 after the whoosh-compat
migration. The unquoted spelling was never parser-native anywhere: v2
rewrote it to explicit bracket ranges app-side before whoosh saw the
string, and the deleted translation layer consumed it itself, so users
and saved views have relied on it continuously while whoosh-compat
deliberately scopes it out of its parser (its DIVERGENCES.md entry 19)
and understands the phrases natively only as quoted values.

parse_user_query now quotes the closed six-phrase vocabulary (previous
week/month/quarter/year, this month/year) when it directly follows a
date field's colon, before parsing. Only quoting happens app-side; every
date computation stays in whoosh-compat's grammar, unlike v2's rewrite,
which computed the ranges itself. Date field names derive from
PUBLIC_FIELDS, the field name matches case-sensitively (the parser's own
field tagging is case-sensitive), the phrase case-insensitively (the
grammar accepts any case in the quoted form), and already-quoted
spellings, TEXT fields, unfielded words and bracketed ranges are
untouched.

The previously xfailed end-to-end regression test now passes as a plain
test, and a new acceptance class pins unquoted == quoted == mixed-case
result sets on a boundary fixture, no-error parsing for the whole
vocabulary across all three date fields, and that "title:previous month"
stays an ordinary text search. docs/usage.md now states the two
spellings are equivalent after a date field.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WMsn6DgzbvSqh1pwy66VVF

docs/usage.md

@@ -923,8 +923,9 @@ produ*name
 
 Matching natural date keywords:
 
-Multi-word date keywords must be quoted (e.g. `added:"previous month"`); an
-unquoted multi-word keyword is not recognized as a date keyword.
+Multi-word date keywords work quoted or unquoted after a date field
+(`added:"previous month"` and `added:previous month` are equivalent);
+elsewhere in a query the same words are treated as ordinary search text.
 
 ```
 added:today

src/documents/search/_query.py

@@ -13,6 +13,7 @@ from whoosh_compat.errors import Diagnostic
 from whoosh_compat.errors import DiagnosticKind
 from whoosh_compat.errors import UnsupportedQueryError
 
+from documents.search._fields import PUBLIC_FIELDS
 from documents.search._registry import get_field_registry
 from documents.search._tokenizer import simple_search_tokens
 
@@ -70,6 +71,59 @@ _REGEX_TIMEOUT: Final[float] = 1.0
 # Uses Unicode properties to cover all blocks including Extension B+ planes.
 _CJK_RE: Final = regex.compile(r"[\p{Han}\p{Hiragana}\p{Katakana}\p{Hangul}]+")
 
+# The closed multi-word date-keyword vocabulary, unchanged since paperless
+# v2's rewrite_natural_date_keywords. whoosh-compat's date grammar
+# understands every one of these natively, but only as a QUOTED value
+# (its DIVERGENCES.md entry 19: unquoted multi-word values split at
+# whitespace, faithfully to whoosh); the unquoted spelling has been
+# honored continuously since the whoosh era by an app-level assist, so
+# _quote_date_keyword_phrases below keeps honoring it by inserting the
+# quotes and nothing else. Single-word keywords (today, yesterday) parse
+# unquoted already and need no entry.
+_DATE_KEYWORD_PHRASES: Final = (
+    "previous week",
+    "previous month",
+    "previous quarter",
+    "previous year",
+    "this month",
+    "this year",
+)
+
+# Field names are case-sensitive (matching the parser's own field
+# tagging); the keyword phrase is case-insensitive (matching the date
+# grammar's leniency for the quoted form). Date fields derived from
+# PUBLIC_FIELDS, never hand-listed.
+_DATE_KEYWORD_PHRASE_RE: Final = regex.compile(
+    r"\b("
+    + "|".join(
+        regex.escape(f.name)
+        for f in PUBLIC_FIELDS
+        if f.kind in (wc.FieldKind.DATE, wc.FieldKind.DATETIME)
+    )
+    + r"):((?i:"
+    + "|".join(_DATE_KEYWORD_PHRASES)
+    + r"))\b",
+)
+
+
+def _quote_date_keyword_phrases(raw_query: str) -> str:
+    """Quote unquoted multi-word date keyword phrases on date fields.
+
+    ``added:previous month`` becomes ``added:"previous month"``; the
+    already-quoted spellings don't match the pattern (the colon must be
+    followed directly by the phrase), and the same words after a TEXT
+    field or standing alone are ordinary text and untouched. Only quoting
+    happens here: every date computation stays in whoosh-compat's
+    grammar, which parses exactly this phrase vocabulary as quoted
+    values. This is deliberately NOT a revival of the deleted
+    translation layer, which computed the ranges app-side.
+    """
+    return _DATE_KEYWORD_PHRASE_RE.sub(
+        r'\1:"\2"',
+        raw_query,
+        timeout=_REGEX_TIMEOUT,
+    )
+
 
 def _has_cjk(text: str) -> bool:
     """Return True if text contains any CJK characters."""
@@ -285,7 +339,10 @@ def parse_user_query(
     """
     Parse user query through whoosh-compat, then blend in fuzzy/CJK clauses.
 
-    1. wc.parse() against the shared FieldRegistry (whoosh grammar -> AST).
+    1. Unquoted multi-word date keyword phrases on date fields are quoted
+       (_quote_date_keyword_phrases) so the historically honored
+       "added:previous month" spelling keeps working; then wc.parse()
+       against the shared FieldRegistry (whoosh grammar -> AST).
     2. Any diagnostics (bad dates/numbers) map to SearchQueryError subclasses
        and raise — the view returns HTTP 400 with every offending field
        listed, not just the first.
@@ -303,6 +360,7 @@ def parse_user_query(
        never went through the pre-whoosh-compat translation layer either.
     """
     registry = get_field_registry(settings.SEARCH_LANGUAGE)
+    raw_query = _quote_date_keyword_phrases(raw_query)
     result = wc.parse(
         raw_query,
         registry=registry,

src/documents/tests/search/test_acceptance.py

@@ -408,3 +408,95 @@ class TestFuzzyBlendSurvivesWhooshGrammar:
             )
             backend.add_or_update(receipt_only)
             assert _matched_ids(backend, "added:today total NOT receipt") == set()
+
+
+class TestUnquotedDateKeywordPhrases:
+    """The unquoted multi-word date keyword spelling (added:previous month)
+    has been honored continuously since the whoosh era, always by an
+    app-level assist, never by any parser: v2 rewrote it to a bracket range
+    before whoosh saw it, and the deleted _translate.py consumed it itself.
+    whoosh-compat scopes the unquoted form out of its parser on purpose
+    (its DIVERGENCES.md entry 19) but understands the quoted form natively,
+    so paperless quotes the closed phrase vocabulary on date fields before
+    parsing. Only quoting happens app-side; every date computation stays in
+    whoosh-compat."""
+
+    @pytest.fixture
+    def period_documents(self, backend: TantivyBackend) -> dict[str, int]:
+        with time_machine.travel(FROZEN_NOW, tick=False):
+            in_may = Document.objects.create(
+                title="May Doc",
+                content="statement",
+                checksum="kw-may",
+                archive_serial_number=910,
+                added=datetime(2026, 5, 20, 12, 0, tzinfo=UTC),
+            )
+            in_june = Document.objects.create(
+                title="June Doc",
+                content="statement",
+                checksum="kw-june",
+                archive_serial_number=911,
+                added=datetime(2026, 6, 10, 12, 0, tzinfo=UTC),
+            )
+            for doc in (in_may, in_june):
+                backend.add_or_update(doc)
+        return {"in_may": in_may.pk, "in_june": in_june.pk}
+
+    @pytest.mark.parametrize(
+        "query",
+        [
+            pytest.param("added:previous month", id="unquoted"),
+            pytest.param('added:"previous month"', id="quoted"),
+            pytest.param("added:Previous Month", id="unquoted-mixed-case"),
+        ],
+    )
+    def test_unquoted_matches_the_same_documents_as_quoted(
+        self,
+        backend: TantivyBackend,
+        period_documents: dict[str, int],
+        query: str,
+    ) -> None:
+        with time_machine.travel(FROZEN_NOW, tick=False):
+            assert _matched_ids(backend, query) == {period_documents["in_may"]}
+
+    @pytest.mark.parametrize(
+        "query",
+        [
+            pytest.param("added:this month", id="this-month"),
+            pytest.param("added:this year", id="this-year"),
+            pytest.param("added:previous week", id="previous-week"),
+            pytest.param("added:previous quarter", id="previous-quarter"),
+            pytest.param("added:previous year", id="previous-year"),
+            pytest.param("created:previous month", id="created-field"),
+            pytest.param("modified:previous month", id="modified-field"),
+        ],
+    )
+    def test_every_phrase_and_date_field_parses_without_error(
+        self,
+        backend: TantivyBackend,
+        period_documents: dict[str, int],
+        query: str,
+    ) -> None:
+        # The whole vocabulary times every date field must at least parse
+        # and search cleanly (no SearchQueryError -> no HTTP 400); exact
+        # window semantics are whoosh-compat's, pinned in its own suite.
+        with time_machine.travel(FROZEN_NOW, tick=False):
+            _matched_ids(backend, query)
+
+    def test_text_field_keyword_words_are_not_rewritten(
+        self,
+        backend: TantivyBackend,
+        period_documents: dict[str, int],
+    ) -> None:
+        # "previous month" after a TEXT field (or unfielded) is ordinary
+        # text, not a date phrase: a title actually containing the words
+        # matches, and the date-window documents do not.
+        with time_machine.travel(FROZEN_NOW, tick=False):
+            wordy = Document.objects.create(
+                title="Notes from the previous month",
+                content="meeting notes",
+                checksum="kw-text",
+                archive_serial_number=912,
+            )
+            backend.add_or_update(wordy)
+            assert _matched_ids(backend, "title:previous month") == {wordy.pk}

src/documents/tests/test_api_search.py

@@ -720,19 +720,6 @@ class TestDocumentSearchApi(DirectoriesMixin, APITestCase):
         self.assertEqual(results[0]["id"], 3)
         self.assertEqual(results[0]["title"], "bank statement 3")
 
-    @pytest.mark.xfail(
-        reason=(
-            "whoosh-compat's DateParserPlugin intentionally drops whoosh's "
-            "'free' undelimited-date tagging mode (see "
-            "whoosh_compat.parser.dateparse.DateParserPlugin docstring), so "
-            "an unquoted multi-word date keyword like 'added:previous month' "
-            "no longer parses -- it now needs quoting ('added:\"previous "
-            "month\"'), unlike 'added:\"previous quarter\"' a few tests down "
-            "which already quotes. CONFIRMED REGRESSION vs whoosh-compat "
-            "migration; see task-10-report.md."
-        ),
-        raises=AssertionError,
-    )
     def test_search_added_previous_month_excludes_next_period_start(self) -> None:
         """
         GIVEN: