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/api/__init__.py

@@ -1,4 +1,9 @@
-"""API blueprint registration."""
+"""API blueprint registration.
+
+This module is imported by the Quart app factory; it just exposes the
+top-level `api_bp` for backward compatibility with FC-1's health route,
+and ALL_BLUEPRINTS for the factory to register sibling blueprints.
+"""
 
 from quart import Blueprint
 
@@ -6,3 +11,9 @@ from . import health
 
 api_bp = Blueprint("api", __name__, url_prefix="/api")
 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]