feat(termine+matrix): Nutzer-Termin-Einreichung (Draft) + Matrix-Notifications

- /termin-melden: öffentliches Formular → /api/termin legt calendar_item als
  Draft an (scoped Write-Key), Admin bestätigt per draft→published
- /api/cms-webhook: empfängt RustyCMS-Webhooks (X-Webhook-Secret) → Matrix
- src/lib/server/matrix.ts: notifyMatrix() Helper
- Form-Proxies (stellungnahme/contact/newsletter/mitmachen) pingen Matrix
- deploy.yml: Matrix/Webhook/Termin-Key Env-Vars

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/routes/api/contact/+server.ts

@@ -2,6 +2,7 @@ import { json } from "@sveltejs/kit";
 import { env as publicEnv } from "$env/dynamic/public";
 import { env as privateEnv } from "$env/dynamic/private";
 import type { RequestHandler } from "./$types";
+import { notifyMatrix } from "$lib/server/matrix";
 import {
   HONEYPOT_FIELDS,
   LIMITS,
@@ -123,6 +124,9 @@ export const POST: RequestHandler = async ({ request, getClientAddress }) => {
   }
 
   if (res.ok) {
+    void notifyMatrix(
+      `✉️ Neue Kontakt-Nachricht von ${payload.name} (${payload.email})${payload.subject ? `: ${payload.subject}` : ""}`,
+    );
     return json({ ok: true }, { status: 201 });
   }
   if (res.status === 429) {