fix(sync): prevent App↔CLI sync permanent wedge

[seibe]

Summary

Fixes a class of bugs where the App or CLI message sync could become permanently stuck ("wedged"), requiring a full app restart to recover.

Related issues

Addresses: #504, #297, #108, #136, #191, #587, #209, #358 — reports of messages not syncing, sessions stuck on "Waiting for messages...", real-time sync failures between devices.

Comparison with similar PRs

• fix: prevent head-of-line blocking in OutgoingMessageQueue #699 — fixes head-of-line blocking in OutgoingMessageQueue by skipping unreleased items. This is a valid fix for one symptom (OutgoingMessageQueue head-of-line blocking causes 'one behind' responses with background tasks #639), but does not address the underlying causes: unbounded retries, missing timeouts, cursor rewind races, and the app fetching all messages from seq 0. This PR addresses the full chain of failure modes that lead to permanent sync wedge, including server-side reverse pagination support.
• feat: add pagination for loading older chat messages #308 (closed) — an earlier attempt at message pagination for the app. This PR provides a more complete implementation with server-side before_seq parameter, mutual exclusion validation, proper test coverage, and integration with the sync engine's cursor management.

Root causes addressed

• Unbounded retries: InvalidateSync and backoff would retry forever on permanent failures (e.g. 404, decryption errors). Now they have configurable maxRetries and enter a recoverable "wedged" state that can be reset by external triggers like socket reconnect.
• Missing timeouts: Socket emitWithAck and HTTP fetch calls could hang indefinitely. All now have explicit timeouts (15s socket, 30s RPC, 60s HTTP).
• Cursor rewind race: sessionLastSeq could be rewound when a slower HTTP response completed after a faster socket update had already advanced the cursor. Now uses monotonic Math.max advancement.
• Initial load fetches all messages: Previously fetched every message from seq 0 on session open. Now uses reverse pagination to fetch only the latest 50, with a "Load older messages" button for backward pagination.
• User messages invisible: Feature-flag gating in typesRaw.ts caused user messages to be dropped depending on the session protocol send flag state. Both legacy and modern user messages are now always normalized.
• Silent server callback omission: The server's update-metadata socket handler could silently return without calling back, leaving the client's emitWithAck hanging until the new timeout fires.

Changes by package

happy-app

• InvalidateSync: Add wedge state, maxRetries, onError callback, and wedge recovery on re-invalidation
• createBackoff: Add maxRetries option with BackoffGaveUpError typed error
• sync.ts: Refactor fetchMessages into fetchLatestMessages (reverse pagination) and fetchForwardMessages (incremental); add fetchOlderMessages for UI-driven backward pagination; monotonic seq advancement; bounded retries on all sync instances
• apiSocket.ts: Add 30s/15s timeouts on emitWithAck, 60s timeout on fetch with AbortController
• typesRaw.ts: Remove feature-flag gating on user message normalization
• ChatList.tsx: Add "Load older messages" button at list top
• storage.ts: Add hasOlderMessages, oldestLoadedSeq, isLoadingOlder fields

happy-cli

• Mirror InvalidateSync and createBackoff changes (wedge state, maxRetries)
• Add 15s timeouts on socket emitWithAck calls in apiSession.ts and apiMachine.ts

happy-server

• v3SessionRoutes.ts: Add before_seq reverse pagination parameter, make after_seq optional, reject both specified simultaneously
• v3SessionRoutes.test.ts: Add tests for reverse pagination, mutual exclusion, and latest-messages default
• sessionUpdateHandler.ts: Send error callback on missing session instead of silently returning
• eventRouter.ts: Add repeatKey field to feed post update type

Test plan

• Server: New tests for reverse pagination and edge cases pass
• App: Open a session with many messages → only latest 50 load initially → "Load older messages" loads more
• App: Kill server mid-sync → sync enters wedged state → reconnect recovers sync automatically
• App: User messages visible in both legacy and session-protocol modes
• CLI: emitWithAck timeout fires correctly on unresponsive server (15s for metadata, 30s for session RPC)