ui/phase-24: push events, turn-ready toast, single SubscribeEvents consumer

Wires the gateway's signed SubscribeEvents stream end-to-end:

- backend: emit game.turn.ready from lobby.OnRuntimeSnapshot on every
  current_turn advance, addressed to every active membership, push-only
  channel, idempotency key turn-ready:<game_id>:<turn>;
- ui: single EventStream singleton replaces revocation-watcher.ts and
  carries both per-event dispatch and revocation detection; toast
  primitive (store + host) lives in lib/; GameStateStore gains
  pendingTurn/markPendingTurn/advanceToPending and a persisted
  lastViewedTurn so a return after multiple turns surfaces the same
  "view now" affordance as a live push event;
- mandatory event-signature verification through ui/core
  (verifyPayloadHash + verifyEvent), full-jitter exponential backoff
  1s -> 30s on transient failure, signOut("revoked") on
  Unauthenticated or clean end-of-stream;
- catalog and migration accept the new kind; tests cover producer
  (testcontainers + capturing publisher), consumer (Vitest event
  stream, toast, game-state extensions), and a Playwright e2e
  delivering a signed frame to the live UI.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

docs/FUNCTIONAL.md

@@ -672,13 +672,19 @@ runtime status, per-player stats). The engine's "game finished"
 report drives the `running → finished` transition ([Section 3.5](#35-cancellation-and-finish))
 and triggers Race Name Directory promotions ([Section 5](#5-race-name-directory)).
 
-The `game.*` notification kinds (`game.started`, `game.turn.ready`,
-`game.generation.failed`, `game.finished`) are reserved in the
-documentation but have **no producer** in the codebase today; the
-notification catalog explicitly omits them (`backend/internal/notification/catalog.go`).
-Adding a producer is purely additive: register the kind in the
-catalog, populate `MailTemplateID` if email fan-out is desired, and
-have the appropriate domain module call `notification.Submit`.
+Among the `game.*` notification kinds, `game.turn.ready` is wired:
+`lobby.Service.OnRuntimeSnapshot` (`backend/internal/lobby/runtime_hooks.go`)
+emits one intent per advancing `current_turn`, addressed to every
+active membership of the game, with idempotency key
+`turn-ready:<game_id>:<turn>` and JSON payload `{game_id, turn}`. The
+catalog routes the intent through the push channel only; email is
+deliberately omitted to avoid per-turn spam.
+
+The remaining `game.*` kinds (`game.started`, `game.generation.failed`,
+`game.finished`) and `mail.dead_lettered` are reserved without a
+producer; adding one is purely additive (register the kind in the
+catalog, extend the migration `CHECK` constraint, and call
+`notification.Submit` from the appropriate domain module).
 
 ### 6.6 Cross-references