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/api/search_handlers.go

@@ -173,12 +173,13 @@ func (s *Server) handleSearch(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
-	// Batch-load thread info for all hits
+	// Batch-load thread info and received_at fallback for all hits
 	hitIDs := make([]string, len(result.Hits))
 	for i, h := range result.Hits {
 		hitIDs[i] = h.ID
 	}
 	threadInfo, _ := s.store.GetThreadInfo(r.Context(), hitIDs)
+	receivedAts := s.store.GetReceivedAts(r.Context(), hitIDs)
 
 	enriched := make([]enrichedHit, 0, len(result.Hits))
 	for _, h := range result.Hits {
@@ -193,6 +194,8 @@ func (s *Server) handleSearch(w http.ResponseWriter, r *http.Request) {
 				eh.Subject = pm.Subject
 				if !pm.Date.IsZero() {
 					eh.Date = pm.Date.UTC().Format(time.RFC3339)
+				} else if t, ok := receivedAts[h.ID]; ok && !t.IsZero() {
+					eh.Date = t.UTC().Format(time.RFC3339)
 				}
 				eh.HasAttachments = len(pm.Attachments) > 0