Store Yoco secrets in the database; add Yoco settings tab

The Yoco secret key and webhook secret now live in AppSetting (AES-256-GCM
encrypted), with .env as a fallback. A new Site Settings → Yoco tab shows
them masked card-style, registers a webhook with Yoco (storing the secret it
returns) or accepts one entered by hand, and lists/deletes the account's
webhooks. Generic settings endpoints never read or write these keys.

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

README.md

@@ -59,8 +59,6 @@ Documentation: see Frontend README for environment and scripts.
   - DATABASE_URL
   - JWT_SECRET
   - PORT (optional; defaults to 3000 if not set)
-  - YOCO_SECRET_KEY (for payments)
-  - YOCO_WEBHOOK_SECRET (to verify webhooks)
   - EMAIL_HOST, EMAIL_PORT, EMAIL_USER, EMAIL_PASS, EMAIL_FROM (for ticket emails)
   - FRONTEND_URL or APP_BASE_URL (used in email links)
 
@@ -69,6 +67,7 @@ Documentation: see Frontend README for environment and scripts.
   Example for default backend: NEXT_PUBLIC_API_URL=http://localhost:5000
 
 - Organisation name, logo, brand colour, and notification emails are configured via the admin panel (Admin → Site Settings) and do not need to be set in .env files.
+- Yoco payments are set up in Admin → Site Settings → Yoco: paste the secret key, then generate the webhook (or enter its secret). YOCO_SECRET_KEY / YOCO_WEBHOOK_SECRET in backend/.env still work as fallbacks.
 
 ### 3) Run the apps