test(conv): fix flaky IMAP backfill test by ordering the channel lookup

[kmonsoe]

What

Fixes the flaky a newly connected mailbox imports only the last backfillDays of existing mail test added in #1130. It failed on #1132 on both attempts, with expected 1 to be 2 on messagesIngested and expected {} to deeply equal { lastUid: 3, uidValidity: '1' } on the cursor.

Why it flaked

imapChannelFor returned rows[0] from a query with no ORDER BY. From the draft_only test onwards, the suite's org holds two IMAP channels: the original support inbox and "Acme Outreach".

Postgres returns unordered rows in heap order, and an UPDATE writes a new tuple version, which can move a row behind its neighbours. The backfill test:

1. looks up the channel, deletes its cursor and sets inbound.backfillDays with an UPDATE;
2. looks the channel up again, and that UPDATE can now make the other channel come back first;
3. ticks that other channel, whose cursor was never reset, so only 1 message is ingested;
4. reads the cursor of the first channel, which was deleted and never rewritten, so it gets {}.

The flip is easy to reproduce on its own:

CREATE TEMP TABLE ch (id text, config jsonb);
INSERT INTO ch VALUES ('support', '{}'), ('outreach', '{}');
SELECT id FROM ch;  -- support, outreach
UPDATE ch SET config = jsonb_set(config, '{backfillDays}', '7') WHERE id = 'support';
SELECT id FROM ch;  -- outreach, support

Fix

Order the helper's query by created_at, id, so every lookup returns the original support channel, which is what these tests already got in the usual case. This changes test code only; the backfill logic in email-adapter.ts is not involved.

Testing

email.integration.test.ts passes 23/23, three runs in a row locally. Typecheck and lint pass.

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