feat(fc3b): migration 0012 + AppSetting model

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

tests/test_migration_0012.py

@@ -0,0 +1,32 @@
+import pytest
+from sqlalchemy import select
+
+from backend.app.models import AppSetting
+
+pytestmark = pytest.mark.integration
+
+
+@pytest.mark.asyncio
+async def test_app_setting_table_round_trip(db):
+    db.add(AppSetting(key="extension_api_key", value="abc123"))
+    await db.flush()
+    row = (await db.execute(
+        select(AppSetting).where(AppSetting.key == "extension_api_key")
+    )).scalar_one()
+    assert row.value == "abc123"
+    assert row.updated_at is not None
+
+
+@pytest.mark.asyncio
+async def test_app_setting_upsert(db):
+    db.add(AppSetting(key="k", value="v1"))
+    await db.flush()
+    row = (await db.execute(
+        select(AppSetting).where(AppSetting.key == "k")
+    )).scalar_one()
+    row.value = "v2"
+    await db.flush()
+    again = (await db.execute(
+        select(AppSetting.value).where(AppSetting.key == "k")
+    )).scalar_one()
+    assert again == "v2"