Replace the fixed 50-message-per-folder header cap with a background,
resumable full-history backfill, and add a user-configurable device-only
retention limit that prunes local mail beyond it without ever deleting from
the server.
- ImapClient.fetchOlderThan pages a folder backwards in bounded batches,
locating the boundary by binary search over message numbers (O(log n) tiny
UID fetches, memory bounded to one batch).
- MailBackfiller + BackfillWorker page each synced folder newest→oldest,
persisting a per-folder boundary in a new backfill_progress table so a run
interrupted by process death / network loss resumes exactly where it stopped.
Runs off the sync mutex, so foreground sync / pull-to-refresh stay responsive.
- MailSyncer now reconciles server deletions only within the recent UID window
(deleteSyncedInWindowNotIn) instead of wiping everything outside the recent
50, so backfilled history survives each foreground sync. A materialized
messages.uid column powers the windowed reconcile and backfill boundary.
- Body/attachment prefetch still honours FetchPolicy (headers first).
- Per-account count/age overrides (nullable) with a global default; 0 = keep
everything (the default, matching #12).
- MailPruner + PruneWorker delete local rows beyond the limit (cascading
attachment rows + on-disk cache), never issuing a server delete. Deletes are
chunked under SQLite's 999-parameter limit.
- Precedence with backfill: backfill pauses (does not complete) at the
retention floor and both jobs share a maintenance mutex, so they never
contend; foreground fetch is also capped by the count so it can't re-download
what pruning just trimmed.
- Settings UI for the global default and per-account override, with copy making
clear it is device-only, not the server.
Room schema v9→v10 (migration + exported schema + MigrationTestHelper test).
GreenMail tests prove the backfill caches >50 and resumes after interruption;
pruning tests cover count/age limits and never touch the server.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
JMR-devandClaude Opus 4.8
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