fix(gallery): drop include_hidden before the similar() call (#141 step 1)

The /similar route splats **filters into similar(), which doesn't take the new
include_hidden kwarg → TypeError → 500 (test_gallery_similar). Drop it like
post_id; similar() has its own presentation exclusion (#1274), so the
gallery-browse flag doesn't apply there.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/gallery.py

@@ -137,7 +137,11 @@ async def similar():
     except (KeyError, ValueError):
         return jsonify({"error": "similar_to query param required"}), 400
     # post_id is the exclusive post-detail view — not a similarity scope.
-    scope = {k: v for k, v in filters.items() if k != "post_id"}
+    # include_hidden is a gallery-browse flag; similar() has its OWN presentation
+    # exclusion (a similarity-quality concern, #1274), so drop it here (#141).
+    scope = {
+        k: v for k, v in filters.items() if k not in ("post_id", "include_hidden")
+    }
     async with get_session() as session:
         svc = GalleryService(session)
         try: