iTip fixes

[tchapi]

Scheduling emails no longer fails

Invitations are sent from inside the PUT that stores the event, and DavisIMipPlugin let its exceptions escape into sabre. sabre's own IMipPlugin never throws. So anything that went wrong while sending came back to the calendar client as a 500 — and the event was never written.

With SMTP unreachable:

PUT /dav/calendars/tchap/default/meeting.ics → 500
Symfony\Component\Mailer\Exception\TransportException
Connection could not be established with host "ssl://localhost:465"
→ calendarobjects with that uri: 0

Same outcome for an attendee address that isn't RFC-compliant (mailto:someone(at)example.org), which is worth noting because the attendee list comes straight from the organiser's client and nothing validates it before new Address(). The error body also handed the client the internal SMTP host.

Both now return 201 with the event saved. The failure is logged and recorded on the event as a SCHEDULE-STATUS the client can surface:

what fails	result
mail transport down	error log, SCHEDULE-STATUS 5.1, event saved
address is not an email address	error log, SCHEDULE-STATUS 5.3, event saved
unsupported iTIP method	warning log, 5.0, no mail — previously left $action undefined and rendered a blank template
recipient isn't mailto:	warning log
unknown PARTSTAT in a REPLY	warning log
OSM tile server unreachable	warning log, invitation sent without the map
sent	info log

The map one is the same class of bug: rendering the location image is a network call on the PUT path, for decoration on an email.