feat(PROJ-58): Indexierung + OCR optional als Cron-Batch statt Dauerbetrieb

Neues config.yml-Feld batch_mode (index/ocr, Default false = unverändertes
Verhalten). Bei batch_mode:true verarbeiten neue Cron-Jobs (index-pending,
ocr-reprocess) die Backlogs in größeren Abständen statt sofort bei jedem
Mail-Import, um Schreiblast auf der Festplatte zu glätten. Zeiten in
/etc/cron.d/archivmail frei anpassbar.

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

internal/storage/storage.go

@@ -1019,6 +1019,42 @@ func (s *Store) IsIndexed(ctx context.Context, id string) (bool, error) {
 	return indexed, err
 }
 
+// UnindexedMail describes one mail awaiting full-text indexing. TenantID is
+// nil when the mail has no tenant assignment (system-level / global).
+type UnindexedMail struct {
+	ID       string
+	TenantID *int64
+}
+
+// GetUnindexedMails returns up to limit mails with indexed_at IS NULL, newest
+// first. limit <= 0 means no limit. Used by the `index-pending` cron batch
+// command (PROJ-58).
+func (s *Store) GetUnindexedMails(ctx context.Context, limit int) ([]UnindexedMail, error) {
+	if s.db == nil {
+		return nil, nil
+	}
+	q := `SELECT id, tenant_id FROM emails WHERE indexed_at IS NULL ORDER BY received_at DESC`
+	args := []interface{}{}
+	if limit > 0 {
+		q += " LIMIT $1"
+		args = append(args, limit)
+	}
+	rows, err := s.db.Query(ctx, q, args...)
+	if err != nil {
+		return nil, fmt.Errorf("storage: get unindexed mails: %w", err)
+	}
+	defer rows.Close()
+	var out []UnindexedMail
+	for rows.Next() {
+		var m UnindexedMail
+		if err := rows.Scan(&m.ID, &m.TenantID); err != nil {
+			continue
+		}
+		out = append(out, m)
+	}
+	return out, rows.Err()
+}
+
 // ── Backfill ──────────────────────────────────────────────────────────────
 
 // Backfill walks the store directory, parses each email, inserts missing DB