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/service.py

@@ -0,0 +1,60 @@
+"""Business logic shared by the HTTP routes and the bulk importer."""
+
+from __future__ import annotations
+
+import hashlib
+import json
+from typing import Any
+
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+from . import models
+
+
+def _content_sha(data: dict[str, Any]) -> str:
+    return hashlib.sha256(json.dumps(data, sort_keys=True).encode("utf-8")).hexdigest()
+
+
+def looks_like_snapshot(data: Any) -> bool:
+    return isinstance(data, dict) and "binary" in data and "types" in data
+
+
+def _apply_metadata(snap: models.Snapshot, data: dict[str, Any]) -> None:
+    binary = data.get("binary", {})
+    snap.binary_name = binary.get("name", "?")
+    snap.engine = binary.get("engine")
+    snap.compiler = binary.get("compiler")
+    snap.schema_version = data.get("schema_version")
+    snap.n_types = len(data.get("types", []))
+    snap.n_methods = len(data.get("methods", []))
+    snap.n_events = len(data.get("events", []))
+    snap.n_fields = len(data.get("fields", []))
+    snap.data = data
+
+
+def _get_or_create_game(db: Session, name: str) -> models.Game:
+    game = db.scalar(select(models.Game).where(models.Game.name == name))
+    if game is None:
+        game = models.Game(name=name)
+        db.add(game)
+        db.flush()
+    return game
+
+
+def import_snapshot(db: Session, data: dict[str, Any], game_name: str | None = None) -> models.Snapshot:
+    """Upsert a snapshot, deduped by the binary's sha256 (falling back to a content hash)."""
+    sha = data.get("binary", {}).get("sha256") or _content_sha(data)
+    snap = db.scalar(select(models.Snapshot).where(models.Snapshot.sha256 == sha))
+    game = _get_or_create_game(db, game_name) if game_name else None
+
+    if snap is None:
+        snap = models.Snapshot(sha256=sha)
+        db.add(snap)
+    _apply_metadata(snap, data)
+    if game is not None:
+        snap.game_id = game.id
+
+    db.commit()
+    db.refresh(snap)
+    return snap