feat(fc2a): add /api/tags endpoints (autocomplete, create, image association)

Thin async blueprint delegating to TagService. Returns 400 with the
TagValidationError message on bad kind/fandom combos so the frontend can
surface the reason. List/add/remove endpoints scoped under
/api/images/<id>/tags follow REST conventions.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/__init__.py

@@ -15,5 +15,5 @@ api_bp.add_url_rule("/health", view_func=health.get_health, methods=["GET"])
 
 def all_blueprints() -> list[Blueprint]:
     from .gallery import gallery_bp
-    # FC-2a additions: tags, settings, import_admin land in later tasks
-    return [api_bp, gallery_bp]
+    from .tags import tags_bp
+    return [api_bp, gallery_bp, tags_bp]