Post event metadata

Purpose: Let a thread root describe an event (when, until, where, link) in its own record, so event pages, the event block and the calendar all read one source. Scenario: A member opens Event in the composer, sets Date, Until, Time and Place, and posts. The root record carries Event-Date, Event-End-Date, Event-Time and Event-Location; the read model copies them onto the thread row; the thread page, board rows and cards show 📅 start – end at time · place · link. Leaving Until empty, or setting it to the start, records a one-day event. Sources: record headers, parser (source unavailable), write path (source unavailable), composer fields (source unavailable), read-model columns, event block (source unavailable). Boundary: Root-only; replies with event headers are rejected. Writing them requires event support; while it is off, event details are not shown and the records are unchanged. Client-built signed records cannot carry event headers; signed events go through prepare_thread. Older releases ignore Event-End-Date and show the first day only.