Implement If-Schedule-Tag-Match support (RFC 6638)

[tobixen]

⚠️ This comment is AI-generated (Claude Sonnet 4.6 via Claude Code) on behalf of tobixen

The infrastructure for Schedule-Tag is half-built: the tag is received and cached, but
If-Schedule-Tag-Match is never sent on PUT requests, leaving attendee PARTSTAT updates
unprotected against racing organizer updates.

Full analysis and suggested implementation steps are in
docs/design/TODO_SCHEDULE_TAG.md.

Summary of work needed

1. _put / _async_put: Accept an extra_headers dict so conditional headers can
   be injected.

2. save(if_schedule_tag_match=True): Currently accepted but silently ignored.
   Wire it through to _put by reading self.props[cdav.ScheduleTag.tag], loading first
   if the tag is not yet cached.

3. _reply_to_invite_request: Already fetches the schedule-tag property but never
   uses it. Should call save(if_schedule_tag_match=True). The current fallback/recursive
   logic also needs cleaning up.

4. Public schedule_tag property on CalendarObjectResource to avoid callers
   importing cdav.

5. ScheduleTagMismatchError (subclass of PutError) for 412 responses so callers
   can handle the re-fetch-and-merge case explicitly.

6. scheduling.schedule-tag compatibility hint — schedule-tag is a SHOULD in the
   RFC, not all servers implement it.

Background

Schedule-Tag solves the attendee-update race condition: without If-Schedule-Tag-Match,
an attendee updating their PARTSTAT can silently overwrite an organizer update that
arrived between the attendee's GET and PUT. The tag is stable across PARTSTAT-only writes,
so it uniquely identifies the scheduling-significant version of the resource.

RFC refs: https://datatracker.ietf.org/doc/html/rfc6638#section-3.2 and section 3.3.

⚠️ This comment is AI-generated (Claude Sonnet 4.6 via Claude Code) on behalf of tobixen