fix(inbox): prevent incorrect Slack message auto-archive

[MaximeGaudin]

Closes #53.

Summary

• bulk_archive_before now requires synced_at < cutoff — a message sent days ago but only synced seconds ago (backfill, daemon restart) is no longer silently archived before the user ever sees it in the inbox.
• Upsert no longer overwrites is_archived on re-sync — removed is_archived = excluded.is_archived from the ON CONFLICT clause so that user archive decisions are preserved across re-syncs (Slack always passes false, which previously un-archived manually archived messages).

Root cause

bulk_archive_before used timestamp < ?1 (the message's original send time) without checking synced_at. A message posted 4 days ago that lands in the DB during a catch-up sync would be immediately caught by any void archive --before targeting the last 2 days — the user never sees it in their inbox.

Separately, the upsert ON CONFLICT clause unconditionally set is_archived = excluded.is_archived, which for Slack is always false. This meant:

1. Manually archived messages were un-archived on re-sync (data loss of user intent)
2. Incorrectly archived messages could never self-heal via re-sync (catch-up only fetches newer messages)

Test plan

• bulk_archive_before_skips_recently_synced_messages — old timestamp + recent synced_at → not archived
• upsert_preserves_user_archived_flag — re-upsert with is_archived: false keeps the archived state
• All existing bulk_archive_before_* tests updated and passing
• Full cargo test green

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