feat(fc2a): add /api/settings/import and /api/system/stats endpoints

Settings GET returns the single-row ImportSettings DB record; PATCH does
selective field updates (only known editable fields are applied; unknown
keys silently ignored). System stats returns image/tag/storage counts,
task status histogram, and the active batch summary in one shot for the
Settings overview tab.

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

tests/test_api_settings.py

@@ -0,0 +1,56 @@
+import pytest
+
+from backend.app import create_app
+
+
+@pytest.fixture
+async def app():
+    return create_app()
+
+
+@pytest.fixture
+async def client(app):
+    async with app.test_client() as c:
+        yield c
+
+
+@pytest.mark.asyncio
+async def test_get_import_settings_defaults(client):
+    resp = await client.get("/api/settings/import")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    # Defaults from the migration:
+    assert body["min_width"] == 0
+    assert body["min_height"] == 0
+    assert body["skip_transparent"] is False
+    assert body["transparency_threshold"] == 0.9
+
+
+@pytest.mark.asyncio
+async def test_patch_import_settings(client):
+    resp = await client.patch(
+        "/api/settings/import",
+        json={"min_width": 500, "skip_transparent": True, "transparency_threshold": 0.7},
+    )
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    assert body["min_width"] == 500
+    assert body["skip_transparent"] is True
+    assert body["transparency_threshold"] == 0.7
+
+
+@pytest.mark.asyncio
+async def test_patch_rejects_non_object(client):
+    resp = await client.patch("/api/settings/import", json=[1, 2, 3])
+    assert resp.status_code == 400
+
+
+@pytest.mark.asyncio
+async def test_system_stats_shape(client):
+    resp = await client.get("/api/system/stats")
+    assert resp.status_code == 200
+    body = await resp.get_json()
+    for key in ("total_images", "total_tags", "storage_bytes", "tasks", "active_batch"):
+        assert key in body
+    for status in ("pending", "queued", "processing", "complete", "skipped", "failed"):
+        assert status in body["tasks"]