feat(chat): backfill transcripts after a feed sync

[bmc08gt]

Chat history was only ever fetched by opening a chat: loadMessages(for:) had exactly one caller (ConversationScreen, on open) and catchUp fired only for the visible chat or a detected live gap. A fresh login — switching accounts included — starts from a per-owner database with no messages in it, so every conversation in the feed sat empty until it was opened by hand.

What changed

• ConversationController.loadFeed() now backfills the conversations the feed reported, four at a time, outside the loading flag — the feed is on screen the moment the conversations apply and the transcripts fill behind it.
• The branch is chosen from the catch-up cursor, which is the only thing that says whether a transcript was ever pulled: a zero cursor takes the newest page, and a seated cursor behind the feed's latest_event_sequence resumes with GetDelta from where it sits. Choosing on "does this chat hold any message" does not work — the feed persists each conversation's last-message preview as a message row before the backfill plans, so that test is true for nearly every chat in a feed the client has otherwise never fetched. Order matters too: a GetDelta(after: 0) would re-pull whole histories and prepend them, knocking an open transcript off the bottom, so delta only runs on a cursor that is already seated.
• Work is deduped by conversation id (the feed loads per type) and skips any chat whose newest page the open screen is already fetching.
• Conversation carries the server's latestEventSequence from the feed payload. It is server truth valid only at fetch time, so it is deliberately not cached — a conversation restored from the local database reports 0, meaning "unknown", not "empty". No schema change: the local cursor is already persisted as ConversationTable.catchupCursor.

Tests

Four new cases in ConversationControllerTests cover the empty-conversation backfill, the cached no-op, the lagging-cursor delta path, and the regression above — a chat holding only the feed's preview row still gets its transcript fetched. Two existing cases were updated for the new premise: RESET_REQUIRED… now forgets the page start()'s backfill fetched, and the old "a reconnect with no conversation open fetches no transcript" is now "a reconnect backfills a chat it surfaces" — that behavior is the point of the change.