Fix: narrow zstd compression level to the conventional -22..22 range

The raw library bounds (-131072, 22) come from an internal zstd
implementation constant (-ZSTD_TARGETLENGTH_MAX), not a meaningful
distinct level — deeper negative values than -22 buy nothing over -22
in practice, and the zstd CLI/community convention only uses -22..22.
Exposing the raw range via --zip-compression-level would let a user
pass a number like -50000 that "validates" but means nothing.

docs/administration.md

@@ -338,7 +338,7 @@ value set in `-zn` or `--zip-name`.
 
 The compression method for the zip can be set with `--zip-compression`
 (`stored`, `deflated` (default), `bzip2`, `lzma`, or `zstd`) and tuned with
-`--zip-compression-level` (deflated: 0–9, bzip2: 1–9, zstd: -131072–22; ignored
+`--zip-compression-level` (deflated: 0–9, bzip2: 1–9, zstd: -22–22; ignored
 for `stored` and `lzma`). Both options require `--zip`.
 
 !!! warning

src/documents/export/compression.py

@@ -28,12 +28,19 @@ if ZSTD is not None:
 
 # Inclusive (min, max) level bounds per method; None => level not applicable.
 # Verified on CPython 3.14.3.
+#
+# zstd's raw library bounds are (-131072, 22)
+# (compression.zstd.CompressionParameter.compression_level.bounds()) — the
+# minimum is an internal implementation constant (-ZSTD_TARGETLENGTH_MAX),
+# not a meaningful distinct "level"; deeper negative values than -22 buy
+# nothing over -22 in practice. We expose the conventional zstd CLI range
+# instead of the raw library bounds.
 LEVEL_BOUNDS: dict[str, tuple[int, int] | None] = {
     "stored": None,
     "deflated": (0, 9),
     "bzip2": (1, 9),
     "lzma": None,
-    "zstd": (-131072, 22),
+    "zstd": (-22, 22),
 }
 
 # zipfile compress_type id -> method name. 93 = current zstd id, 20 = legacy

src/documents/management/commands/document_exporter.py

@@ -213,7 +213,7 @@ class Command(CryptMixin, PaperlessCommand):
             default=None,
             help=(
                 "Compression level for the export zip (requires --zip). "
-                "deflated: 0-9, bzip2: 1-9, zstd: -131072..22; ignored for "
+                "deflated: 0-9, bzip2: 1-9, zstd: -22..22; ignored for "
                 "stored/lzma."
             ),
         )

src/documents/tests/export/test_compression.py

@@ -78,6 +78,8 @@ class TestLevelError:
             ("deflated", 9),
             ("bzip2", 1),
             ("bzip2", 9),
+            ("zstd", -22),
+            ("zstd", 22),
             ("deflated", None),
             ("stored", None),
         ],
@@ -100,6 +102,8 @@ class TestLevelError:
             ("deflated", -1),
             ("bzip2", 0),
             ("bzip2", 10),
+            ("zstd", -23),
+            ("zstd", 23),
         ],
     )
     def test_out_of_range_levels_return_message(