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/db.py

@@ -0,0 +1,53 @@
+"""Database engine/session wiring. SQLite by default (zero setup), Postgres-ready via DATABASE_URL.
+
+The engine/session factory is process-global and (re)built by `configure()`, so tests can point it
+at a temporary database before creating the app.
+"""
+
+from __future__ import annotations
+
+import os
+from collections.abc import Iterator
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import DeclarativeBase, Session, sessionmaker
+
+DEFAULT_URL = "sqlite:///./ams.db"
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+_engine = None
+_SessionLocal: sessionmaker | None = None
+
+
+def configure(url: str | None = None) -> None:
+    global _engine, _SessionLocal
+    url = url or os.environ.get("DATABASE_URL", DEFAULT_URL)
+    connect_args = {"check_same_thread": False} if url.startswith("sqlite") else {}
+    _engine = create_engine(url, connect_args=connect_args, future=True)
+    _SessionLocal = sessionmaker(bind=_engine, autoflush=False, expire_on_commit=False, class_=Session)
+
+
+def init_db() -> None:
+    from . import models  # noqa: F401  - register mappers before create_all
+    if _engine is None:
+        configure()
+    Base.metadata.create_all(_engine)
+
+
+def get_session() -> Session:
+    if _SessionLocal is None:
+        configure()
+    return _SessionLocal()
+
+
+def get_db() -> Iterator[Session]:
+    """FastAPI dependency yielding a request-scoped session."""
+    db = get_session()
+    try:
+        yield db
+    finally:
+        db.close()