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/diff.py

@@ -0,0 +1,30 @@
+from __future__ import annotations
+
+from typing import Any
+
+from fastapi import APIRouter, Depends, HTTPException, Query
+from sqlalchemy.orm import Session
+
+from ...diff import compute_diff, filter_by_owner
+from ...snapshot import Snapshot
+from .. import models
+from ..db import get_db
+
+router = APIRouter(tags=["diff"])
+
+
+@router.get("/diff")
+def get_diff(
+    old: int = Query(..., description="older snapshot id"),
+    new: int = Query(..., description="newer snapshot id"),
+    owner: str | None = Query(None, description="restrict to one class, e.g. CMC_Animo"),
+    db: Session = Depends(get_db),
+) -> dict[str, Any]:
+    a = db.get(models.Snapshot, old)
+    b = db.get(models.Snapshot, new)
+    if a is None or b is None:
+        raise HTTPException(404, "snapshot not found")
+    diff = compute_diff(Snapshot(a.data), Snapshot(b.data))
+    if owner:
+        diff = filter_by_owner(diff, owner)
+    return diff