Mark All Read

Bulk-mark every message in this group as read by the caller.

The GET handler auto-records reads for the page it returns, but inboxes show a per-conversation unread count that's driven by messages older than the viewport. A dedicated mark-all endpoint lets the client clear that badge in one round-trip instead of paginating backwards.

Inserts MessageRead rows for every previously-unread, non-soft-deleted, not-authored-by-caller message in the conv. Idempotent: re-calling on an already-read conv is a no-op.

Returns the number of new rows written so clients can update their local unread state.