feat(fc2c-i): showcase random-sample service and endpoint

tests/test_api_showcase.py

@@ -0,0 +1,35 @@
+import pytest
+
+from backend.app import create_app
+from backend.app.models import ImageRecord
+
+pytestmark = pytest.mark.integration
+
+
+@pytest.fixture
+async def client():
+    app = create_app()
+    async with app.test_client() as c:
+        yield c
+
+
+@pytest.mark.asyncio
+async def test_showcase_returns_images(client, db):
+    for i in range(8):
+        db.add(ImageRecord(
+            path=f"/images/sc/{i}.jpg", sha256=f"c{i:063d}",
+            size_bytes=1, mime="image/jpeg", width=10, height=10,
+            origin="imported_filesystem", integrity_status="unknown",
+        ))
+    await db.flush()
+    await db.commit()
+    resp = await client.get("/api/showcase?limit=4")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert "images" in body and len(body["images"]) <= 4
+
+
+@pytest.mark.asyncio
+async def test_showcase_rejects_bad_limit(client):
+    resp = await client.get("/api/showcase?limit=oops")
+    assert resp.status_code == 400