fix(happy-app): debounce encryption-race message recovery (companion to #701)

[cjb0001]

Summary

Companion follow-up to slopus#701: add debounced recovery scheduling for session-encryption race paths so message sync does not repeatedly thrash retries when encryption is temporarily unavailable.

Why this follow-up

slopus#701 already addresses wedge behavior broadly (bounded retries, timeouts, cursor monotonicity, reverse pagination). One remaining edge case is transient session-encryption unavailability during fetch/update handling, which can still trigger repeated immediate invalidations.

This PR adds a tiny, focused recovery debounce to smooth that path.

Changes (on top of slopus#701)

• Add per-session messageRecoveryTimers map.
• Add scheduleMessageRecovery(sessionId, delayMs=500) helper.
• In fetchMessages, when session encryption is missing:
  • schedule a delayed recovery instead of throwing/retrying immediately.
• In new-message path, when encryption is still missing after session refresh:
  • schedule delayed recovery instead of immediate reinvalidate.
• Clear recovery timer on delete-session cleanup.

File changed

• packages/happy-app/sources/sync/sync.ts

Validation

• corepack yarn --cwd packages/happy-app typecheck ✅

Base

• Base branch: fix/sync-wedge (mirror of upstream PR fix(sync): prevent App↔CLI sync permanent wedge slopus/happy#701 head)