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/routes/snapshots.py

@@ -0,0 +1,41 @@
+from __future__ import annotations
+
+from typing import Any
+
+from fastapi import APIRouter, Body, Depends, HTTPException, Query
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+from .. import models, schemas, service
+from ..db import get_db
+
+router = APIRouter(prefix="/snapshots", tags=["snapshots"])
+
+
+@router.post("", response_model=schemas.SnapshotOut, status_code=201)
+def create_snapshot(
+    data: dict[str, Any] = Body(..., description="a full engine-surface snapshot.json"),
+    game: str | None = Query(None, description="link to / create this game by name"),
+    db: Session = Depends(get_db),
+) -> models.Snapshot:
+    if not service.looks_like_snapshot(data):
+        raise HTTPException(422, "body is not an engine-surface snapshot (missing binary/types)")
+    return service.import_snapshot(db, data, game)
+
+
+@router.get("", response_model=list[schemas.SnapshotOut])
+def list_snapshots(
+    game_id: int | None = Query(None), db: Session = Depends(get_db)
+) -> list[models.Snapshot]:
+    q = select(models.Snapshot)
+    if game_id is not None:
+        q = q.where(models.Snapshot.game_id == game_id)
+    return list(db.scalars(q.order_by(models.Snapshot.id)))
+
+
+@router.get("/{snapshot_id}", response_model=schemas.SnapshotDetail)
+def get_snapshot(snapshot_id: int, db: Session = Depends(get_db)) -> models.Snapshot:
+    snap = db.get(models.Snapshot, snapshot_id)
+    if snap is None:
+        raise HTTPException(404, "snapshot not found")
+    return snap