v1.15.1: team reschedule write-back, embed theming, invite subjects

Patch release with three field-reported fixes on top of 1.15.0. No schema changes, no new features, no config changes. Drop-in upgrade.

Fixed

• Team reschedules reach the calendar again (#159, #160, thanks @hugo-fasone) — Rescheduling a booking on a team event type updated the database but pushed an ICS whose ORGANIZER was the event type owner, while the event had been created with the assigned member as organizer. Google CalDAV rejects an organizer change on an existing event with 403 Forbidden, so the calendar silently kept the old time while guest and host were told the reschedule succeeded. Reschedule and both approval flows now resolve the organizer from the booking itself (assigned member for round-robin, full roster for collective, owner for personal event types), matching what the booking flow wrote at creation.
• No more black frame around themed embeds (#165, thanks @pycanis) — An inline embed pinned to ?theme=light showed its light content inside a black frame for visitors whose OS is in dark mode (and vice versa), because the color-scheme meta always advertised light dark and the iframe canvas followed the OS. The meta now follows the pinned embed theme; auto-themed embeds and regular pages are unchanged.
• Neutral subjects on guest emails carrying a calendar invite (#157, #158) — Exchange iMIP processing titles the guest's calendar appointment after the email Subject rather than the ICS SUMMARY, so prefixes like "Confirmed:" leaked into the event title and bounced back in accept/decline replies. The three guest-facing emails that attach a METHOD:REQUEST ICS now use a plain "event, date" subject in all six locales; the confirmed/rescheduled wording stays in the email body.

Internal

• 797 tests, all green (new regression test covering organizer resolution for assigned, collective, and personal bookings)

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