Take in invitations that arrive by mail

A watched mailbox is a trust boundary this app did not have: anyone who knows
the address can hand the server an iCalendar file. So a mail can do exactly one
thing — create a row in event_invites. No calendar read path touches that
table, and an invitation becomes an event only when a person accepts it.

Routing is by the ATTENDEE address, never the From header, which a sender can
claim freely. Addresses are matched against users.email plus a dedicated table,
so forwarded mail still finds its owner: the invitation names the original
address, not the mailbox it landed in.

An accepted invitation gets a fresh UID; the organiser's goes to external_uid.
LocalEvent.uid is globally unique, so reusing it would let a stranger's mail
collide with an event already there. That case has a test, because it is the
one that would be a real problem.

parse_ics now reads METHOD, SEQUENCE, the organiser address, the ATTENDEE list
and RECURRENCE-ID. All additive — existing callers read the keys they knew.
METHOD sits on the VCALENDAR, so it is read outside the component loop; that is
the only reliable way to tell an invitation from a plain calendar file. A VEVENT
carrying RECURRENCE-ID is skipped rather than applied, since treating one
occurrence as the series would move every other date.

The mailbox job registers only when IMAP_HOST is set: an unconditional job would
log every few minutes that it has nothing to do, and test_scheduler asserts the
exact job set. Credentials come from the environment, which keeps the one secret
that opens a whole mailbox out of a table an endpoint could return.

Replies to the organiser are the next step; accepting and declining work without
them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/main.py

@@ -18,7 +18,7 @@ sys.path.insert(0, str(Path(__file__).parent))
 
 import scheduler
 from database import Base, engine
-from routers import admin_router, attachments_router, auth_router, birthdays_router, caldav_router, dav_router, google_router, groups_router, homeassistant_router, ical_router, local_router, oidc_router, profile_router, settings_router, users_router
+from routers import admin_router, attachments_router, auth_router, birthdays_router, caldav_router, dav_router, google_router, groups_router, homeassistant_router, ical_router, invites_router, local_router, oidc_router, profile_router, settings_router, users_router
 
 logging.basicConfig(level=logging.INFO)
 
@@ -460,6 +460,7 @@ app.include_router(ical_router.router, prefix="/api/ical", tags=["ical"])
 app.include_router(google_router.router, prefix="/api/google", tags=["google"])
 app.include_router(homeassistant_router.router, prefix="/api/homeassistant", tags=["homeassistant"])
 app.include_router(admin_router.router, prefix="/api/instance", tags=["instance"])
+app.include_router(invites_router.router, prefix="/api/invites", tags=["invites"])
 # CalDAV publishing lives at root scope (no /api prefix) and must be registered
 # before the SPA catch-all so /dav/... isn't swallowed by the index fallback.
 app.include_router(dav_router.router, tags=["dav"])