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/schemas.py

@@ -0,0 +1,43 @@
+"""Pydantic request/response models."""
+
+from __future__ import annotations
+
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict
+
+
+class GameCreate(BaseModel):
+    name: str
+    notes: str | None = None
+
+
+class GameOut(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+    id: int
+    name: str
+    notes: str | None = None
+
+
+class SnapshotOut(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+    id: int
+    game_id: int | None
+    binary_name: str
+    sha256: str
+    engine: str | None
+    compiler: str | None
+    schema_version: int | None
+    n_types: int
+    n_methods: int
+    n_events: int
+    n_fields: int
+    created_at: datetime
+
+
+class SnapshotDetail(SnapshotOut):
+    data: dict
+
+
+class GameDetail(GameOut):
+    snapshots: list[SnapshotOut] = []