What does this PR do?

Adds the ability to invite team members as optional guests on calendar invites for team event types.

The use case: you want a teammate — a shadow, observer, backup — to get the calendar invite, but you don’t want their availability to block the booking. Think CC’ing someone on a meeting.

Fixes #18947

/claim #18947

How it works end-to-end

1. New M2M relation between EventType and User via _optional_guest_team_members join table
2. UI toggle in Advanced settings → “Invite team members as optional guests” with a multi-select picker
3. All 6 calendar integrations handled natively:
   • Google → optional: true, responseStatus: needsAction
   • Exchange 2013/2016/generic → OptionalAttendees.Add()
   • Feishu/Lark → is_optional: true
4. Email dedup so optional guests aren’t duplicated if they’re already attendees
5. Optional guests are excluded from availability conflict checks entirely

What I considered

• Previous PRs (#22651, #22127) had stale code and merge conflicts — wrote this from scratch using current patterns
• Each calendar service uses its native optional attendee mechanism rather than hacking around it
• Prisma implicit M2M since we just need user IDs with no extra metadata

Mandatory Tasks (DO NOT REMOVE)

• I have self-reviewed the code (A decent size PR without self-review might be rejected).
• I have updated the developer docs in /docs if this PR makes changes that would require a documentation change. N/A — no docs change needed.
• I confirm automated tests are in place that prove my fix is effective or that my feature works.

How should this be tested?

1. Create/edit a team event type → Advanced tab
2. Toggle on “Invite team members as optional guests” → pick 1-2 members → Save
3. Book the event from the public page
4. Check the calendar invite — selected members should show as optional attendees
5. Verify non-team event types don’t show the toggle

Edge cases worth checking

• Toggle on → select members → toggle off → should clear selection on save
• Select a member who’s also a host → should not be duplicated (dedup logic)
• Exchange calendars → member should be in OptionalAttendees, not RequiredAttendees