feat(gallery): composable scroll filter (multi-tag AND, media, sort)

Phase 1 backend for the gallery filter bar. Extends scroll/timeline/jump
from a single mutually-exclusive filter to a composable one:

- tag_ids: image must carry ALL of them (one correlated EXISTS per tag —
  AND, no row multiplication), replacing the single-tag JOIN.
- artist_id composes with tags; media_type ('image'|'video') narrows by
  mime; post_id stays the exclusive post-detail path.
- sort ('newest'|'oldest') flips the effective_date/id cursor comparison
  and ordering; the cursor value is unchanged (direction comes from the
  request). jump_cursor honors sort too.
- Shared _apply_scope helper applied across scroll/timeline/jump so the
  timeline sidebar reflects the filtered set. API _parse_filters parses
  tag_id (comma list), artist_id, media, sort.

Tests: multi-tag AND, media filter, sort reversal (service + API);
post_id-excludes-others; single tag_id back-compat.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/test_api_gallery.py

@@ -59,3 +59,21 @@ async def test_jump_requires_year_month(client):
 async def test_image_detail_404_when_missing(client):
     resp = await client.get("/api/gallery/image/99999")
     assert resp.status_code == 404
+
+
+@pytest.mark.asyncio
+async def test_scroll_sort_param_reverses(client, db):
+    await _seed(db, 3)
+    newest = await (await client.get("/api/gallery/scroll?limit=10&sort=newest")).get_json()
+    oldest = await (await client.get("/api/gallery/scroll?limit=10&sort=oldest")).get_json()
+    ids_new = [i["id"] for i in newest["images"]]
+    ids_old = [i["id"] for i in oldest["images"]]
+    assert ids_old == list(reversed(ids_new))
+
+
+@pytest.mark.asyncio
+async def test_scroll_media_param(client, db):
+    await _seed(db, 2)  # only image/jpeg seeded
+    resp = await client.get("/api/gallery/scroll?limit=10&media=video")
+    assert resp.status_code == 200
+    assert (await resp.get_json())["images"] == []