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/demo.go

@@ -1,6 +1,7 @@
 package api
 
 import (
+	"errors"
 	"log/slog"
 	"net/http"
 	"strings"
@@ -75,3 +76,18 @@ func (s *Server) startDemoResetTicker() {
 		s.resetDemo()
 	}
 }
+
+// A public demo must not send anything to the outside (FLAW-33): no alarm mails, Discord or webhook
+// calls, no test messages. The senders refuse, and the routes that only exist to send or to change
+// server settings answer with 403 right away. All visitors share the demo accounts, so whatever
+// would lock the others out (passwords, ending sessions, deleting accounts) is refused as well.
+var errDemoOutgoing = errors.New("In der Demo deaktiviert")
+
+func (s *Server) notInDemo(h http.HandlerFunc) http.HandlerFunc {
+	if !s.cfg.DemoMode {
+		return h
+	}
+	return func(w http.ResponseWriter, r *http.Request) {
+		writeError(w, http.StatusForbidden, "In der Demo deaktiviert")
+	}
+}