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>

ams/api/app.py

@@ -0,0 +1,30 @@
+"""FastAPI application factory.
+
+Run with uvicorn's factory mode (no import-time DB side effects):
+
+    uvicorn ams.api.app:create_app --factory --reload
+"""
+
+from __future__ import annotations
+
+from fastapi import FastAPI
+
+from .. import __version__
+from .db import configure, init_db
+from .routes import diff, games, snapshots
+
+
+def create_app(database_url: str | None = None) -> FastAPI:
+    configure(database_url)
+    init_db()
+
+    app = FastAPI(title="ams — engine surface catalog", version=__version__)
+    app.include_router(games.router)
+    app.include_router(snapshots.router)
+    app.include_router(diff.router)
+
+    @app.get("/health", tags=["meta"])
+    def health() -> dict[str, str]:
+        return {"status": "ok", "version": __version__}
+
+    return app