fix(security): demo sends nothing, hidden SMTP password, body limits (FLAW-33..38)

- Demo: senders for mail, Discord and webhooks refuse, no alarm ticker;
  test alarm, SMTP test, server settings, backups, account and session
  changes of the shared demo accounts answer 403; the session list only
  shows the own device. The UI shows the notices and disables the controls.
- SMTP password is never sent back (smtpPassSet), empty keeps it.
- ?token= on webhook ingest accepts only API keys.
- Request bodies capped at 2 MB (sync 8 MB, imports 101 MB).
- Deleting an account removes its server backups.
- Backup downloads, exports and import uploads get 30 minutes.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/handlers_account.go

@@ -135,6 +135,7 @@ func (s *Server) handleImportObsidian(w http.ResponseWriter, r *http.Request) {
 // handleExportMarkdown streams an Obsidian-compatible ZIP of the user's workspace (no temp files).
 func (s *Server) handleExportMarkdown(w http.ResponseWriter, r *http.Request) {
 	userID := auth.GetUserID(r.Context())
+	allowLongTransfer(w)
 	w.Header().Set("Content-Type", "application/zip")
 	w.Header().Set("Content-Disposition", fmt.Sprintf(`attachment; filename="mindsift-vault-%s.zip"`, time.Now().Format("2006-01-02")))
 	if err := s.store.WriteMarkdownVault(userID, w); err != nil {
@@ -145,6 +146,7 @@ func (s *Server) handleExportMarkdown(w http.ResponseWriter, r *http.Request) {
 
 func (s *Server) handleExport(w http.ResponseWriter, r *http.Request) {
 	userID := auth.GetUserID(r.Context())
+	allowLongTransfer(w)
 	exportData, err := s.store.ExportUserData(userID)
 	if err != nil {
 		writeError(w, http.StatusInternalServerError, err.Error())