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/models/tag.py

@@ -48,6 +48,10 @@ class TagKind(StrEnum):
 # content. `wip` is real art: only the training pipelines exclude it.
 SYSTEM_TAG_NAMES = ("wip", "banner", "editor screenshot")
 PRESENTATION_SYSTEM_TAGS = ("banner", "editor screenshot")
+# `wip` marks real-but-unfinished art. It's kept in the gallery's own "similar"
+# results (#1274), but the Explore rabbit-hole opts to hide it (exclude_wip) so a
+# browse doesn't keep surfacing work-in-progress (operator, 2026-07-08).
+WIP_SYSTEM_TAG = "wip"
 
 image_tag = Table(
     "image_tag",