feat: add CalDAV support for ProtonMail Calendar

[bobbiejaxn]

Implements CalDAV proxy bridging ProtonMail Calendar API to standard CalDAV protocol (RFC 4791).

What's new

• caldav/caldav.go (452 lines): Full caldav.Backend implementation

  • CalendarHomeSetPath, ListCalendars, GetCalendar
  • ListCalendarObjects, GetCalendarObject, QueryCalendarObjects
  • PutCalendarObject (create/update events with PGP encryption)
  • DeleteCalendarObject
  • Per-calendar event cache with EventRefreshCalendar invalidation

• protonmail/calendar.go: Extended with 7 API methods + PGP crypto

  • CalendarEventCard.Read() / NewEncryptedCalendarEventCard()
  • GetCalendar, CreateCalendar, DeleteCalendar
  • GetCalendarEvent, CreateCalendarEvent, UpdateCalendarEvent, DeleteCalendarEvent

• protonmail/events.go: EventRefreshCalendar + EventCalendarEvent

• cmd/hydroxide/main.go: New --caldav-port flag, caldav serve command

• go.mod / go.sum: Added go-webdav/caldav dependency

Implementation notes

• CardDAV implementation used as template (carddav/carddav.go)
• go-webdav/caldav library handles all CalDAV protocol handling
• PGP encryption follows same pattern as ContactCard crypto
• Calendar creation is forbidden (matches CardDAV address book pattern)

Closes #207

[bobbiejaxn]

Ping @emersion — this PR is a complete CalDAV implementation that mirrors the existing CardDAV backend pattern. It's mergeable with a clean diff. The other open CalDAV PRs (#282 has merge conflicts / dirty state, #329 is minimal with only ListCalendars) are less comprehensive. Happy to address any review feedback.

[bobbiejaxn]

I've rebased this PR onto the latest master. The merge conflict in has been resolved (kept the newer upstream dependency versions alongside the dependency needed for CalDAV). CI should be re-triggering shortly. Would appreciate a review when you have capacity.

[bobbiejaxn]

I've rebased this PR onto the latest master. The merge conflict in go.mod has been resolved (kept the newer upstream dependency versions alongside the rrule-go dependency needed for CalDAV). CI should be re-triggering shortly. Would appreciate a review when you have capacity.