feat(PROJ-20): GoBD-konforme Nutzer-Löschung mit IMAP-Cleanup und Warn-Dialog

- imap/store.go: DeleteByOwner() – löscht alle IMAP-Konten eines Nutzers
- api/server.go: handleDeleteUser lädt Nutzername vor Löschung, ruft DeleteByOwner, schreibt erweitertes Audit-Log (username, role, IMAP-Count, GoBD-Hinweis)
- admin/page.tsx: confirm() ersetzt durch Dialog mit GoBD-Hinweis, Deaktivieren-Option (empfohlen) und endgültigem Löschen (destruktiv)
- features/PROJ-20-nutzer-loeschung.md: Feature-Spec angelegt
- features/INDEX.md: PROJ-20 eingetragen, Next ID → PROJ-21

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

internal/api/server.go

@@ -385,22 +385,42 @@ func (s *Server) handleDeleteUser(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	// Fetch user info before deletion for audit log and IMAP cleanup
+	target, err := s.users.GetByID(id)
+	if err != nil {
+		writeError(w, http.StatusNotFound, "user not found")
+		return
+	}
+
 	if err := s.users.DeleteSafe(id); err != nil {
 		if err.Error() == "userstore: cannot delete last admin" {
 			writeError(w, http.StatusConflict, "cannot delete the last active admin")
 			return
 		}
-		writeError(w, http.StatusNotFound, err.Error())
+		writeError(w, http.StatusInternalServerError, err.Error())
 		return
 	}
 
+	// Remove all IMAP accounts that belonged to this user
+	imapDeleted := 0
+	if s.imapStore != nil {
+		if n, err := s.imapStore.DeleteByOwner(r.Context(), target.Username); err != nil {
+			s.logger.Warn("delete user: could not remove IMAP accounts", "user", target.Username, "err", err)
+		} else {
+			imapDeleted = n
+		}
+	}
+
 	sess := sessionFromCtx(r.Context())
 	s.audlog.Log(audit.Entry{
 		EventType: audit.EventUserMgmt,
 		Username:  sess.Username,
 		IPAddress: remoteIP(r),
-		Detail:    fmt.Sprintf("deleted user %d", id),
-		Success:   true,
+		Detail: fmt.Sprintf(
+			"deleted user %d (%s, role=%s); %d IMAP account(s) removed; emails retained per GoBD",
+			id, target.Username, target.Role, imapDeleted,
+		),
+		Success: true,
 	})
 
 	writeJSON(w, http.StatusOK, map[string]bool{"ok": true})