Enhancement: document_index reindex --if-needed; simplify Docker startup

Add --if-needed flag to `document_index reindex`: checks _needs_rebuild()
(schema version + language sentinels) and skips if index is up to date.
Safe to run on every upgrade or startup.

Simplify Docker init-search-index script to unconditionally call
`reindex --if-needed` — schema/language change detection is now fully
delegated to Python. Removes the bash index_version and language file
tracking entirely; Tantivy's own sentinels are the source of truth.

Update docs: bare metal upgrade step uses --if-needed; Docker note
updated to describe the new always-runs-but-skips-if-current behaviour.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/documents/management/commands/document_index.py

@@ -21,10 +21,26 @@ class Command(PaperlessCommand):
             default=False,
             help="Wipe and recreate the index from scratch (only used with reindex).",
         )
+        parser.add_argument(
+            "--if-needed",
+            action="store_true",
+            default=False,
+            help=(
+                "Skip reindex if the index is already up to date. "
+                "Checks schema version and search language sentinels. "
+                "Safe to run on every startup or upgrade."
+            ),
+        )
 
     def handle(self, *args, **options):
         with transaction.atomic():
             if options["command"] == "reindex":
+                if options.get("if_needed"):
+                    from documents.search._schema import _needs_rebuild
+
+                    if not _needs_rebuild(settings.INDEX_DIR):
+                        self.stdout.write("Search index is up to date.")
+                        return
                 if options.get("recreate"):
                     from documents.search import wipe_index