fix: sync participants immediately when session is created from event

[devin-ai-integration]

Summary

Fixes an issue where the event metadata chip would show empty participants when a session was created from an event. The root cause was a timing issue: sessions created via getOrCreateSessionForEventId didn't get participants until the next calendar sync cycle (up to 60 seconds later).

This PR adds immediate participant syncing when a new session is created from an event by:

1. Adding fetchEventParticipants to fetch participants for a specific event from Apple Calendar
2. Adding syncParticipantsForSession to sync participants for a single session (reuses existing computeSessionParticipantChanges logic)
3. Calling the sync as a fire-and-forget async operation after session creation

Review & Testing Checklist for Human

• Test the main flow: Create a session from a calendar event with multiple participants and verify participants appear immediately in the metadata chip (not after 60 seconds)
• Test edge cases: Event without participants, event not yet synced to local store, calendar without proper permissions
• Verify periodic sync still works: The existing 60-second sync cycle should continue to work and not conflict with this immediate sync
• Check silent failure paths: The syncEventParticipantsToSession function has multiple early returns with no logging - verify this is acceptable behavior

Recommended Test Plan

1. Open the app with a connected Apple Calendar
2. Click on an upcoming event with participants to create a session
3. Verify participants appear immediately in the event metadata chip
4. Wait for the periodic sync (60s) and verify no duplicate participants are created

Notes

• The sync is fire-and-forget (void syncEventParticipantsToSession(...)) - failures are silent
• Uses store as MergeableStore type assertion due to TinyBase type differences
• The existing computeSessionParticipantChanges logic respects excluded participants and only modifies source: "auto" mappings

Link to Devin run: https://app.devin.ai/sessions/cf67ab4469c746a79a12846bf1f6e56e
Requested by: @ComputelessComputer

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[devin-ai-integration]

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