fix: Date-Parsing-Fallback für nicht-standard MTA-Datumsformate

mailparser: weitere Layouts (Timezone +02:00 mit Doppelpunkt, ohne Sekunden)
storage: GetReceivedAts() für Batch-Lookup von received_at
search_handlers: received_at als Fallback wenn pm.Date.IsZero()

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

internal/storage/storage.go

@@ -682,6 +682,31 @@ func (s *Store) Delete(id string) error {
 	return nil
 }
 
+// GetReceivedAts returns the received_at timestamp for each mail ID in the
+// provided slice. Used as a date fallback when the email Date header cannot
+// be parsed. Missing IDs are silently omitted from the result map.
+func (s *Store) GetReceivedAts(ctx context.Context, ids []string) map[string]time.Time {
+	if s.db == nil || len(ids) == 0 {
+		return nil
+	}
+	// pgx supports $1 = []string via ANY
+	rows, err := s.db.Query(ctx,
+		`SELECT id, received_at FROM emails WHERE id = ANY($1)`, ids)
+	if err != nil {
+		return nil
+	}
+	defer rows.Close()
+	result := make(map[string]time.Time, len(ids))
+	for rows.Next() {
+		var id string
+		var t time.Time
+		if err := rows.Scan(&id, &t); err == nil {
+			result[id] = t
+		}
+	}
+	return result
+}
+
 // Purge deletes all mails whose retain_until has passed.
 // Returns the number of successfully deleted mails.
 // Mails that fail to delete (e.g. file missing) are skipped silently.