feat(explore): exclude WIP-tagged work from the Explore rabbit-hole

Explore's neighbour grid (/api/gallery/similar → gallery_service.similar) now
takes an Explore-only exclude_wip flag that drops `wip` system-tagged images
from the candidates, alongside the banner/editor presentation tags. The
gallery's own "similar" button is unchanged (keeps wip, #1274) — only the
Explore store passes exclude_wip=1. The anchor itself may still be a WIP; only
neighbours are filtered.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/gallery.py

@@ -145,6 +145,9 @@ async def similar():
         filters, _sort = _parse_filters()
     except (KeyError, ValueError):
         return jsonify({"error": "similar_to query param required"}), 400
+    # Explore passes exclude_wip=1 to also drop work-in-progress from the
+    # rabbit-hole; the gallery's own "similar" button omits it (keeps wip, #1274).
+    exclude_wip = request.args.get("exclude_wip") in ("1", "true", "True")
     # post_id is the exclusive post-detail view — not a similarity scope.
     # include_hidden is a gallery-browse flag; similar() has its OWN presentation
     # exclusion (a similarity-quality concern, #1274), so drop it here (#141).
@@ -154,7 +157,8 @@ async def similar():
     async with get_session() as session:
         svc = GalleryService(session)
         try:
-            images = await svc.similar(image_id=similar_to, limit=limit, **scope)
+            images = await svc.similar(
+                image_id=similar_to, limit=limit, exclude_wip=exclude_wip, **scope)
         except ValueError as exc:
             return jsonify({"error": str(exc)}), 400
     if images is None: