Use OpenMeet as calendar availability layer (replaces direct Google Calendar integration)

[zhiganov]

OpenMeet already handles calendar OAuth (Google, Outlook) and exposes availability APIs. If users connect their calendar on OpenMeet, avails can pull their busy times without its own Google OAuth setup.

OpenMeet endpoints:

• POST /api/external-calendar/events — get calendar events for a date range (with names, times)
• POST /api/availability/conflicts — get conflicts for a time range
• POST /api/availability/check — check specific time slot
• Auth: ATProto service auth (same Bluesky login)

What this replaces:

• Our entire client/src/lib/googleCalendar.js
• The VITE_GOOGLE_CLIENT_ID env var and Google Cloud OAuth setup
• The Google consent screen, demo video requirement, testing mode hassle
• Server-side Google Calendar OAuth with persistent tokens #6 Server-side Google Calendar OAuth (OpenMeet handles this)
• Add Microsoft/Outlook calendar support #3 Microsoft/Outlook support (OpenMeet already supports it)

Flow:

1. User signs into avails with Bluesky (already works)
2. If they have an OpenMeet account with calendar connected, fetch their busy times via OpenMeet API
3. Show on the grid — same as current Google Calendar overlay, but from OpenMeet
4. If they haven't connected a calendar on OpenMeet, show a link to do so

Depends on: #31 (OpenMeet integration basics)

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Status (2026-04-03): Implementation complete. Proper ATProto service auth flow: PDS signs JWT with aud: did:web:api.openmeet.net, lxm: net.openmeet.auth, exchange at OpenMeet's service-auth endpoint. Blocked by OpenMeet 500 error — reported as OpenMeet-Team/openmeet-api#573. Google Calendar fallback works in the meantime.

[zhiganov]

Shipped 2026-04-23. Same unblocker as #31 — once bsky actually granted the rpc:net.openmeet.auth?aud=* scope on a fresh consent (after client_id rotation in #73/#74), calendar availability via POST /api/external-calendar/events started working end-to-end. Confirmed live on poll pages for signed-in users with OpenMeet accounts linked to their Bluesky ("Calendar connected via OpenMeet — busy times shown in pink").