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>

tests/test_api_gallery.py

@@ -0,0 +1,70 @@
+from datetime import datetime, timedelta, timezone
+
+import pytest
+
+from backend.app import create_app
+from backend.app.models import ImageRecord
+
+
+@pytest.fixture
+async def app():
+    return create_app()
+
+
+@pytest.fixture
+async def client(app):
+    async with app.test_client() as c:
+        yield c
+
+
+async def _seed(db, count: int = 3):
+    base = datetime.now(timezone.utc)
+    for i in range(count):
+        r = ImageRecord(
+            path=f"/images/x/{i}.jpg",
+            sha256=f"x{i:063d}",
+            size_bytes=1, mime="image/jpeg", width=1, height=1,
+            origin="imported_filesystem", integrity_status="unknown",
+        )
+        r.created_at = base - timedelta(minutes=i)
+        db.add(r)
+    await db.flush()
+    await db.commit()
+
+
+@pytest.mark.asyncio
+async def test_scroll_returns_images(client, db):
+    await _seed(db)
+    resp = await client.get("/api/gallery/scroll?limit=10")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert len(body["images"]) == 3
+    assert "next_cursor" in body
+    assert "date_groups" in body
+
+
+@pytest.mark.asyncio
+async def test_scroll_rejects_bad_limit(client):
+    resp = await client.get("/api/gallery/scroll?limit=notanumber")
+    assert resp.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_timeline_endpoint(client, db):
+    await _seed(db)
+    resp = await client.get("/api/gallery/timeline")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert sum(b["count"] for b in body) == 3
+
+
+@pytest.mark.asyncio
+async def test_jump_requires_year_month(client):
+    resp = await client.get("/api/gallery/jump")
+    assert resp.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_image_detail_404_when_missing(client):
+    resp = await client.get("/api/gallery/image/99999")
+    assert resp.status_code == 404