fix(billing): wire up Stripe checkout with real price IDs and env vars

- Replace 8 placeholder price IDs (price_starter_monthly, etc.) with real
  Stripe test-mode price IDs created via API
- Add STRIPE_SECRET_KEY, STRIPE_WEBHOOK_SECRET, NEXT_PUBLIC_BILLING_PROVIDER,
  and NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY to docker-compose.yml (build args + runtime)
- Add NEXT_PUBLIC_BILLING_PROVIDER ARG/ENV to Dockerfile
- Enable team and personal account billing (was 'false')
- Created Stripe webhook endpoint for production URL
- Created 4 Stripe products (Starter/Pro/Verband/Enterprise) with monthly+yearly prices

Checkout was crashing because:
1. STRIPE_SECRET_KEY was missing → Zod validation failed at createStripeClient()
2. STRIPE_WEBHOOK_SECRET was missing → same Zod schema rejection
3. NEXT_PUBLIC_BILLING_PROVIDER was unset → BillingProviderSchema.parse() failed
4. Price IDs were placeholders, not real Stripe price_xxx IDs

docker-compose.yml

@@ -330,6 +330,9 @@ services:
         # Browser-side Supabase URL — goes through external domain (Traefik → Kong)
         NEXT_PUBLIC_SUPABASE_URL: ${API_EXTERNAL_URL:-http://localhost:8000}
         NEXT_PUBLIC_SUPABASE_PUBLIC_KEY: ${SUPABASE_ANON_KEY}
+        # Stripe (build-time)
+        NEXT_PUBLIC_BILLING_PROVIDER: stripe
+        NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY: ${NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY}
     restart: unless-stopped
     depends_on:
       supabase-kong:
@@ -352,12 +355,17 @@ services:
       NEXT_PUBLIC_ENABLE_THEME_TOGGLE: 'true'
       NEXT_PUBLIC_ENABLE_TEAM_ACCOUNTS: 'true'
       NEXT_PUBLIC_ENABLE_TEAM_ACCOUNTS_CREATION: 'true'
-      NEXT_PUBLIC_ENABLE_TEAM_ACCOUNTS_BILLING: 'false'
-      NEXT_PUBLIC_ENABLE_PERSONAL_ACCOUNT_BILLING: 'false'
+      NEXT_PUBLIC_ENABLE_TEAM_ACCOUNTS_BILLING: 'true'
+      NEXT_PUBLIC_ENABLE_PERSONAL_ACCOUNT_BILLING: 'true'
       NEXT_PUBLIC_ENABLE_NOTIFICATIONS: 'true'
       NEXT_PUBLIC_ENABLE_FISCHEREI: 'true'
       NEXT_PUBLIC_ENABLE_MEETING_PROTOCOLS: 'true'
       NEXT_PUBLIC_ENABLE_VERBANDSVERWALTUNG: 'true'
+      # Stripe (runtime)
+      NEXT_PUBLIC_BILLING_PROVIDER: stripe
+      NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY: ${NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY}
+      STRIPE_SECRET_KEY: ${STRIPE_SECRET_KEY}
+      STRIPE_WEBHOOK_SECRET: ${STRIPE_WEBHOOK_SECRET}
 
 volumes:
   supabase-db-data: