Add game-state rollback (Phase 1)

Full-state JSON snapshots let a scene be rolled back to an earlier
action. All game state lives in game_id-scoped tables, so a checkpoint
is just a serialized dump of the gameplay rows and rollback restores
it -- sidestepping deterministic replay (the deck shuffle is
materialized into state and captured verbatim).

- Checkpoint table + GameEvent.checkpoint_id (tri-state) +
  Game.rollback_timeline_version (models.py, Alembic migration).
- crud_rollback.py: serialize/apply/capture/seal-purge/rollback.
- Capture is driven by the broadcast middleware: snapshot per action
  while in the `scene` phase, seal+purge otherwise. Confined to the
  current scene; older scenes' checkpoints are purged at scene end.
- POST /game/{gid}/player/{pid}/rollback (routes_rollback.py),
  server-enforced for Admins and Deep players.
- EventLog.svelte: per-event rollback buttons + timeline reconciliation.
- Remove the orphaned /scene/rollback per-card-play undo (dead code
  since d7f8483, never wired up; superseded by full-state rollback).

Phase 1 truncates the future immediately; the greyed/undoable redo is
deferred to Phase 2.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

TODO.md

@@ -1,5 +1,5 @@
 ## Major features
-- [ ] **Rollback of game state.** This may require some updates to how game state is handled in the backend (my understanding is that the frontend always gets full state updates rather than deltas, so it should be unaffected). Feel free to propose significant overhauls/refactors to enable this cleanly, I am not attached to the current database architecture at all. Also feel free to ask clarifying questions before going ahead with implementation. I think a clean UI for this would involve the event log. Each event gets a "rollback" button to roll back the state of the game to a checkpoint just after that event was posted. Until a new event is posted, the later entries in the log remain in a greyed out state so that the rollback can be undone if it was accidental. These buttons appear to the admin player if dev mode is on, and also to Deep players, but only going back as far as the start of the current scene if they aren't the admin.
+- [~] **Rollback of game state.** *Phase 1 implemented* (full-state JSON snapshots in a `Checkpoint` table, captured per action by the broadcast middleware, restored by `crud_rollback.rollback_to_checkpoint`; scene-confined; server-enforced for Admins and Deep players; event-log buttons in `EventLog.svelte`). **Phase 1 truncates the future immediately (no undo).** *Phase 2 (remaining):* the greyed-out / undoable redo UX — roll back without discarding the future, grey later entries, and only commit the truncation when a new action is posted. The schema is already forward-compatible for this (add `Game.rollback_head_checkpoint_id`).
 
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