feat(gallery): faceted filter params + /facets counts endpoint (Phase 2 backend)

Extend the composable gallery filter with platform / untagged / no_artist /
date_from / date_to, AND-composed with the existing tag/artist/media/sort
params and threaded through scroll, timeline, and jump_cursor.

Add GalleryService.facets() + GET /api/gallery/facets returning live counts
scoped to the current filter with per-group minus-self semantics: platform
counts (COUNT(DISTINCT image) incl. a null unsourced bucket), curation-flag
counts (untagged / no_artist), and effective_date min/max bounds. The
UNSOURCED_PLATFORM sentinel makes filesystem-imported content reachable via
the platform facet.

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

tests/test_api_gallery.py

@@ -77,3 +77,40 @@ async def test_scroll_media_param(client, db):
     resp = await client.get("/api/gallery/scroll?limit=10&media=video")
     assert resp.status_code == 200
     assert (await resp.get_json())["images"] == []
+
+
+@pytest.mark.asyncio
+async def test_facets_endpoint(client, db):
+    await _seed(db, 3)  # filesystem images: no artist, no tags, no platform
+    resp = await client.get("/api/gallery/facets")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["total"] == 3
+    assert body["untagged"] == 3
+    assert body["no_artist"] == 3
+    assert "platforms" in body
+    assert body["date_min"] is not None
+    assert body["date_max"] is not None
+
+
+@pytest.mark.asyncio
+async def test_facets_rejects_bad_date(client):
+    resp = await client.get("/api/gallery/facets?date_from=notadate")
+    assert resp.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_scroll_untagged_param(client, db):
+    await _seed(db, 2)
+    resp = await client.get("/api/gallery/scroll?untagged=1&limit=10")
+    assert resp.status_code == 200
+    assert len((await resp.get_json())["images"]) == 2
+
+
+@pytest.mark.asyncio
+async def test_scroll_date_from_param(client, db):
+    await _seed(db, 2)  # all created ~now
+    # A future date_from excludes everything.
+    resp = await client.get("/api/gallery/scroll?date_from=2099-01-01&limit=10")
+    assert resp.status_code == 200
+    assert (await resp.get_json())["images"] == []