feat(PROJ-45): IMAP per-folder UID-tracking, UIDVALIDITY-check + reindex OCR protection

- scheduler.go: BUG-1 fix — preserve stored uid_validity when server returns 0
- scheduler.go: BUG-2 fix — replace inline switch with DecideResync() call
- scheduler.go: SetAuditLogger wired; imap_uidvalidity_reset audit event
- cmd_reindex.go: read existing attachment_text before IndexSync to prevent
  Manticore REPLACE INTO from wiping OCR text written by the OCR worker

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/imap/scheduler.go

@@ -302,13 +302,16 @@ func (s *Scheduler) syncFolder(
 		serverUIDValidity = selectData.UIDValidity
 	}
 
-	effectiveLastUID := state.LastUID
+	// BUG-2 fix: use DecideResync instead of inline switch to keep the
+	// branching logic in one place and avoid drift with the unit-tested path.
+	decision := DecideResync(state.LastUID, state.UIDValidity, serverUIDValidity)
+	effectiveLastUID := decision.EffectiveStart
 
-	switch {
-	case serverUIDValidity == 0:
+	if serverUIDValidity == 0 {
 		log.Warn("uidvalidity zero from server, keeping stored last_uid",
 			"folder", folder, "stored_uid_validity", state.UIDValidity)
-	case state.UIDValidity != 0 && state.UIDValidity != serverUIDValidity:
+	}
+	if decision.UIDValidReset {
 		// Mailbox was rebuilt server-side — UIDs restart at 1. We must
 		// resync the entire folder from scratch. PROJ-32 message-id
 		// deduplication prevents duplicate writes.
@@ -327,7 +330,6 @@ func (s *Scheduler) syncFolder(
 					acc.ID, folder, state.UIDValidity, serverUIDValidity),
 			})
 		}
-		effectiveLastUID = 0
 	}
 
 	var uids []imapv2.UID
@@ -359,14 +361,22 @@ func (s *Scheduler) syncFolder(
 		maxUID   uint32 = effectiveLastUID
 	)
 
-	// PROJ-45: even when there are 0 new UIDs, persist the (possibly new)
-	// uid_validity so a later UIDVALIDITY change can still be detected.
+	// BUG-1 fix: when the server returns UIDVALIDITY=0 (misbehaving server),
+	// keep the stored valid value so future mismatch detection still works.
+	// Writing 0 would silently disable the UIDVALIDITY guard for this folder.
+	persistedValidity := serverUIDValidity
+	if serverUIDValidity == 0 {
+		persistedValidity = state.UIDValidity
+	}
+
+	// PROJ-45: even when there are 0 new UIDs, persist the uid_validity
+	// so a later UIDVALIDITY change can still be detected.
 	defer func() {
 		newState := FolderState{
 			AccountID:   acc.ID,
 			Folder:      folder,
 			LastUID:     maxUID,
-			UIDValidity: serverUIDValidity,
+			UIDValidity: persistedValidity,
 		}
 		if err := s.store.UpsertFolderState(ctx, newState); err != nil {
 			log.Warn("imap scheduler: persist folder state failed",