feat(artist): post_count on the artist overview response — drives the Posts/Gallery default-tab fallback in the upcoming ArtistView redesign

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

tests/test_api_artist.py

@@ -29,6 +29,30 @@ async def test_artist_overview_ok(client, db):
     body = await resp.get_json()
     assert body["name"] == "Mira"
     assert body["image_count"] == 0
+    assert body["post_count"] == 0
+
+
+@pytest.mark.asyncio
+async def test_artist_overview_post_count(client, db):
+    from backend.app.models import Post, Source
+
+    a = Artist(name="Lyra", slug="lyra")
+    db.add(a)
+    await db.flush()
+    s = Source(
+        artist_id=a.id, platform="patreon",
+        url="https://patreon.com/cw/lyra", enabled=True,
+    )
+    db.add(s)
+    await db.flush()
+    db.add(Post(source_id=s.id, external_post_id="p1"))
+    db.add(Post(source_id=s.id, external_post_id="p2"))
+    await db.flush()
+    await db.commit()
+    resp = await client.get("/api/artist/lyra")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["post_count"] == 2
 
 
 @pytest.mark.asyncio