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/models.py

@@ -0,0 +1,51 @@
+"""ORM models: a Game has many Snapshots. The full snapshot.json is stored verbatim in `data`
+(JSON / JSONB); axis counts are denormalised for cheap listing. Diffing reads `data` back through
+the existing ams.diff engine, so the DB never has to mirror the snapshot schema."""
+
+from __future__ import annotations
+
+from datetime import datetime, timezone
+
+from sqlalchemy import ForeignKey, JSON, String, UniqueConstraint
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from .db import Base
+
+
+def _utcnow() -> datetime:
+    return datetime.now(timezone.utc)
+
+
+class Game(Base):
+    __tablename__ = "games"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    name: Mapped[str] = mapped_column(String, unique=True, index=True)
+    notes: Mapped[str | None] = mapped_column(String, default=None)
+
+    snapshots: Mapped[list["Snapshot"]] = relationship(
+        back_populates="game", cascade="all, delete-orphan")
+
+
+class Snapshot(Base):
+    __tablename__ = "snapshots"
+    __table_args__ = (UniqueConstraint("sha256", name="uq_snapshot_sha256"),)
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    game_id: Mapped[int | None] = mapped_column(ForeignKey("games.id"), default=None, index=True)
+
+    binary_name: Mapped[str] = mapped_column(String)
+    sha256: Mapped[str] = mapped_column(String, index=True)
+    engine: Mapped[str | None] = mapped_column(String, default=None)
+    compiler: Mapped[str | None] = mapped_column(String, default=None)
+    schema_version: Mapped[int | None] = mapped_column(default=None)
+
+    n_types: Mapped[int] = mapped_column(default=0)
+    n_methods: Mapped[int] = mapped_column(default=0)
+    n_events: Mapped[int] = mapped_column(default=0)
+    n_fields: Mapped[int] = mapped_column(default=0)
+
+    created_at: Mapped[datetime] = mapped_column(default=_utcnow)
+    data: Mapped[dict] = mapped_column(JSON)
+
+    game: Mapped["Game | None"] = relationship(back_populates="snapshots")