feat(fc2b): add /api/images/<id>/suggestions endpoints

GET (ranked, category-grouped), accept (→ apply + allowlist; enqueues
retro-apply when newly added), alias (create alias + accept canonical),
dismiss (per-image rejection). Thin blueprint over SuggestionService +
AllowlistService. Tests marked integration, eager Celery.

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

backend/app/api/__init__.py

@@ -17,5 +17,13 @@ def all_blueprints() -> list[Blueprint]:
     from .gallery import gallery_bp
     from .import_admin import import_admin_bp
     from .settings import settings_bp
+    from .suggestions import suggestions_bp
     from .tags import tags_bp
-    return [api_bp, gallery_bp, tags_bp, settings_bp, import_admin_bp]
+    return [
+        api_bp,
+        gallery_bp,
+        tags_bp,
+        settings_bp,
+        import_admin_bp,
+        suggestions_bp,
+    ]