Add FastAPI catalog backend (games/snapshots/diff) + tests

Modular-monolith backend over SQLAlchemy (SQLite by default, Postgres-ready
via DATABASE_URL). The full snapshot.json is stored verbatim; diffing reads it
back through the ams.diff engine, so the DB never mirrors the snapshot schema.

- ams.api.db/models/schemas/service : Game 1-N Snapshot, sha256-deduped upsert
- routes: POST/GET /games, POST/GET /snapshots (import, deduped), GET /diff
          (?old&new[&owner]) running compute_diff on stored snapshots, /health
- ams.api.importer : bulk CLI loader (python -m ams.api.importer --game ...)
- run: uvicorn ams.api.app:create_app --factory

11 tests pass (6 diff + 5 API via TestClient over the golden pair). Smoke-tested
live on uvicorn: import -> /snapshots -> /diff returns the BlooMoo deltas.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -15,3 +15,6 @@ __pycache__/
 
 # macOS
 .DS_Store
+
+# Local SQLite catalog DB
+*.db