fix: write through outbound calendar updates to the local event cache

[PixelJonas]

What does this change?

Rescheduling a meeting pushes the new time to the external calendar provider (CalDAV/Google/Outlook) synchronously — availability checks are always correct — but this outbound path never updated the local provider_calendar_event cache table or broadcast a cache-update event, unlike the inbound sync path, which does both.

Since the calendar-view dedup logic prefers the cache row over the live Meeting whenever the two are linked, the calendar grid kept showing the pre-reschedule time until the next inbound sync cycle happened to reconcile it (15-90 minutes later, whenever that provider's poll/webhook next fired).

CalendarEventSync.update/2 now, after a successful outbound push:

• looks up the linked cache row by provider_event_id for OAuth providers, falling back to uid for CalDAV — mirroring ExternalCalendarChanges.find_linked_meeting/3's existing lookup order
• writes through the pushed start/end time and content fields via a new narrow ProviderCalendarEventQueries.update_after_outbound_push/2
• broadcasts the change the same way inbound sync already does

A failed outbound push never touches the cache, and a missing cache row (e.g. the first outbound push before any inbound sync has run yet) is a no-op.

Covered by 4 new tests: CalDAV write-through, OAuth provider_event_id-keyed write-through, no-op when there's no cache row, and no write on push failure.

Checklist

• Commits are signed off with git commit -s (required — CI enforces the DCO; see CONTRIBUTING)
• Tests pass locally (mix test), if relevant