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

Dockerfile

@@ -22,6 +22,7 @@ ARG NEXT_PUBLIC_ENABLE_FISCHEREI=true
 ARG NEXT_PUBLIC_ENABLE_MEETING_PROTOCOLS=true
 ARG NEXT_PUBLIC_ENABLE_VERBANDSVERWALTUNG=true
 ARG NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=
+ARG NEXT_PUBLIC_BILLING_PROVIDER=stripe
 ENV NEXT_PUBLIC_CI=${NEXT_PUBLIC_CI}
 ENV NEXT_PUBLIC_SITE_URL=${NEXT_PUBLIC_SITE_URL}
 ENV NEXT_PUBLIC_SUPABASE_URL=${NEXT_PUBLIC_SUPABASE_URL}
@@ -31,6 +32,7 @@ ENV NEXT_PUBLIC_ENABLE_FISCHEREI=${NEXT_PUBLIC_ENABLE_FISCHEREI}
 ENV NEXT_PUBLIC_ENABLE_MEETING_PROTOCOLS=${NEXT_PUBLIC_ENABLE_MEETING_PROTOCOLS}
 ENV NEXT_PUBLIC_ENABLE_VERBANDSVERWALTUNG=${NEXT_PUBLIC_ENABLE_VERBANDSVERWALTUNG}
 ENV NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY=${NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY}
+ENV NEXT_PUBLIC_BILLING_PROVIDER=${NEXT_PUBLIC_BILLING_PROVIDER}
 RUN pnpm --filter web build
 
 # --- Run ---