feat(PROJ-18): E-Mail Integritätsprüfung (SHA-256 Verifikation)

- Storage: VerifyIntegrity, GetAllIDs, GetVerifyStatus + DB-Spalten
- main: Hintergrund-Worker alle 5 Minuten (beim Start sofort: 40/40 OK)
- API: verify_ok + verified_at in GET /api/mails/{id} Antwort
- Frontend: Grüner Haken / graues X / rotes X in Mail-Ansicht

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

internal/api/server.go

@@ -548,6 +548,17 @@ func (s *Server) handleGetMail(w http.ResponseWriter, r *http.Request) {
 		dateStr = pm.Date.UTC().Format(time.RFC3339)
 	}
 
+	// Verify status
+	vs, _ := s.store.GetVerifyStatus(r.Context(), id)
+	var verifyOK interface{} = nil
+	var verifiedAt interface{} = nil
+	if vs.VerifyOK != nil {
+		verifyOK = *vs.VerifyOK
+	}
+	if vs.VerifiedAt != nil {
+		verifiedAt = vs.VerifiedAt.UTC().Format(time.RFC3339)
+	}
+
 	writeJSON(w, http.StatusOK, map[string]interface{}{
 		"id":          id,
 		"from":        pm.From,
@@ -560,6 +571,8 @@ func (s *Server) handleGetMail(w http.ResponseWriter, r *http.Request) {
 		"body_plain":  pm.TextBody,
 		"raw_headers": extractRawHeaders(raw),
 		"attachments": attachments,
+		"verify_ok":   verifyOK,
+		"verified_at": verifiedAt,
 	})
 }