feat(fc2a): add /api/gallery endpoints (scroll, timeline, jump, image detail)

Thin Quart blueprint that delegates to GalleryService. Cursor-based scroll
returns images + a date_groups summary so the SPA can render year-month
headers without re-grouping client-side. Timeline returns year-month
buckets for the sidebar jump nav. Jump returns a cursor positioned at a
specific year-month so the gallery can scroll to historical periods.

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

backend/app/__init__.py

@@ -4,7 +4,7 @@ import logging
 
 from quart import Quart
 
-from .api import api_bp
+from .api import all_blueprints
 from .config import get_config
 from .frontend import frontend_bp
 
@@ -15,7 +15,9 @@ def create_app() -> Quart:
 
     app = Quart(__name__)
     app.secret_key = cfg.secret_key
-    app.register_blueprint(api_bp)
+
+    for bp in all_blueprints():
+        app.register_blueprint(bp)
     # Registered last so /api/* routes win over the SPA catch-all.
     app.register_blueprint(frontend_bp)