feat(gateway): unsigned gateway.heartbeat keeps Safari push streams alive

Browser fetch-streaming layers close response bodies they consider
idle after roughly 15-30 s without incoming bytes. Safari is the
most aggressive, but the symptom matters everywhere: a quiet
SubscribeEvents stream (lobby, between turns, mailbox empty) gets
torn down by the browser, the EventStream singleton reconnects with
backoff, and any push event that fires inside the reconnect window
is lost because `push.Hub` queues are not persisted across
subscription closes. The user-visible failure mode is the
intermittent "Fetch API cannot load … due to access control checks"
console error (a misleading WebKit symptom — CORS headers are
actually present) plus missed turn-ready / mail-received toasts.

Server-side fix: a silence-based heartbeat at the
`authenticatedPushStreamService` wrapper layer. After the signed
`gateway.server_time` bootstrap event, gateway wraps the bound
stream with `heartbeatingStream`. Every tail Send (fan-out, future
variants) resets the silence timer; when the timer elapses, a
goroutine emits `gateway.heartbeat` with only `EventType` set —
everything else stays at proto3 defaults, so the wire frame is
~45 bytes amortised. A `sendMu` serialises the heartbeat goroutine
with tail Sends because grpc.ServerStream.Send is not goroutine-safe.

The heartbeat is intentionally UNSIGNED: heartbeats carry no
payload, dispatch to no handler on the client, and an injected
heartbeat trivially causes no user-visible state change. TLS still
protects the wire and real events keep the signed envelope
unchanged. Documented in `docs/ARCHITECTURE.md` § 15 alongside the
per-scale bandwidth projection (100…100 000 clients × 15…60 s).

Config: new `GATEWAY_PUSH_HEARTBEAT_INTERVAL` (default `15s`,
`0s` disables). Telemetry: new
`gateway.push.heartbeats_sent{outcome}` counter so operators can
budget bandwidth and spot a sudden `outcome=error` bump as an
upstream-failing-before-flush signal.

Client (`ui/frontend/src/api/events.svelte.ts`): early `continue`
on `event.eventType === "gateway.heartbeat"` before `verifyEvent`,
`verifyPayloadHash`, or dispatch — empty signature would otherwise
trip SignatureError and reconnect. A leading heartbeat still flips
`connectionStatus` to `connected` and resets backoff, because
receiving one is proof the stream is healthy.

Tests:
- `push_heartbeat_test.go`: unit tests for the wrapper — zero
  interval returns nil, heartbeat fires after silence, real Send
  resets the timer, Stop / context-cancel halt the goroutine,
  Send errors propagate.
- `server_test.go`: integration tests through the full gateway
  pipeline — heartbeat fires after the configured silence window,
  zero interval keeps the stream silent.
- `config_test.go`: default applied, env-override parsed,
  negative value rejected.
- `events.test.ts`: heartbeat skipped before verification + not
  dispatched to handlers; leading heartbeat still flips
  `connectionStatus` to `connected`.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

gateway/README.md

@@ -294,7 +294,17 @@ the stream to the verified `user_id` and `device_session_id`, sends one
 signed `gateway.server_time` bootstrap event whose FlatBuffers payload carries
 `server_time_ms`, registers the active stream in the in-memory `PushHub`, and
 then forwards signed client-facing events consumed from the configured client
-event Redis stream. User-targeted events fan out to every active stream for
+event Redis stream. After the bootstrap, the stream is wrapped with a
+silence-based heartbeat: when no real event has been forwarded for
+`GATEWAY_PUSH_HEARTBEAT_INTERVAL` (default `15s`, set to `0s` to disable),
+the gateway emits an unsigned `gateway.heartbeat` event so browser
+fetch-streaming layers (Safari is the most aggressive) keep the response
+body open and pending push events are not lost into the client-side
+reconnect window. Each emission is counted by the
+`gateway.push.heartbeats_sent{outcome}` metric; see
+[`docs/ARCHITECTURE.md` § 15](../docs/ARCHITECTURE.md#15-transport-security-model-gateway-boundary)
+for the bandwidth projection and the reasoning behind the unsigned
+envelope. User-targeted events fan out to every active stream for
 that user. Session-targeted events fan out only to streams whose
 `user_id` and `device_session_id` both match the event target. Each active
 stream uses a bounded in-memory queue; when that queue overflows, only the