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>

cmd/archivmail/cmd_reindex.go

@@ -129,6 +129,19 @@ func runReindex(args []string) {
 		}
 
 		idx := idxMgr.ForTenant(tenantID)
+
+		// Preserve existing OCR text — IndexSync uses REPLACE which would
+		// otherwise wipe attachment_text written by the OCR worker.
+		// Only read from the index when the DB confirms OCR has run (ocr_chars>0).
+		_, ocrChars, _ := mailStore.GetOCRMeta(ctx, id)
+		if ocrChars > 0 {
+			if reader, ok := idx.(index.AttachmentTextReader); ok {
+				if existing, err := reader.GetAttachmentText(id); err == nil && existing != "" {
+					doc.AttachmentText = existing
+				}
+			}
+		}
+
 		if err := idx.IndexSync(doc); err != nil {
 			logger.Warn("reindex: index failed", "id", id, "err", err)
 			errors++