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>

tests/test_api.py

@@ -0,0 +1,70 @@
+"""API tests against a temp SQLite DB, exercising import -> list -> diff over the golden pair."""
+
+from __future__ import annotations
+
+import json
+from pathlib import Path
+
+import pytest
+
+pytest.importorskip("fastapi")
+from fastapi.testclient import TestClient  # noqa: E402
+
+from ams.api.app import create_app  # noqa: E402
+
+SNAP_DIR = Path(__file__).resolve().parents[1] / "snapshots"
+PIKLIB = SNAP_DIR / "PIKLIB8.dll.snapshot.json"
+BLOOMOO = SNAP_DIR / "bloomoodll.dll.snapshot.json"
+
+pytestmark = pytest.mark.skipif(not PIKLIB.exists(), reason="golden snapshots not present")
+
+
+@pytest.fixture()
+def client(tmp_path) -> TestClient:
+    app = create_app(database_url="sqlite:///{0}/api.db".format(tmp_path))
+    return TestClient(app)
+
+
+def _load(path: Path) -> dict:
+    with open(path, "r", encoding="utf-8") as fh:
+        return json.load(fh)
+
+
+def test_health(client: TestClient):
+    assert client.get("/health").json()["status"] == "ok"
+
+
+def test_import_list_and_dedup(client: TestClient):
+    r = client.post("/snapshots", params={"game": "Reksio i UFO"}, json=_load(PIKLIB))
+    assert r.status_code == 201
+    first_id = r.json()["id"]
+    assert r.json()["engine"] == "Piklib" and r.json()["n_types"] == 40
+
+    # re-importing the same binary (same sha256) updates in place, not a duplicate row
+    r2 = client.post("/snapshots", json=_load(PIKLIB))
+    assert r2.json()["id"] == first_id
+    assert len(client.get("/snapshots").json()) == 1
+
+    # the game was auto-created and linked
+    games = client.get("/games").json()
+    assert [g["name"] for g in games] == ["Reksio i UFO"]
+
+
+def test_invalid_snapshot_rejected(client: TestClient):
+    assert client.post("/snapshots", json={"not": "a snapshot"}).status_code == 422
+
+
+def test_diff_endpoint(client: TestClient):
+    a = client.post("/snapshots", params={"game": "Reksio i UFO"}, json=_load(PIKLIB)).json()["id"]
+    b = client.post("/snapshots", params={"game": "Reksio i Kapitan Nemo"}, json=_load(BLOOMOO)).json()["id"]
+
+    diff = client.get("/diff", params={"old": a, "new": b}).json()
+    added = {t["script_name"] for t in diff["types"]["added"]}
+    assert {"GRBUFFER", "INTERNET"} <= added
+
+    animo = client.get("/diff", params={"old": a, "new": b, "owner": "CMC_Animo"}).json()
+    assert "GETFPS" in {m["name"] for m in animo["methods"]["added"]}
+
+
+def test_diff_missing_snapshot_404(client: TestClient):
+    assert client.get("/diff", params={"old": 999, "new": 998}).status_code == 404