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

apps/web/config/billing.config.ts

@@ -33,7 +33,7 @@ export default createBillingSchema({
           interval: 'month',
           lineItems: [
             {
-              id: 'price_starter_monthly',
+              id: 'price_1THsqKKttnWb7SsFttMu9VzG',
               name: 'Starter',
               cost: 29,
               type: 'flat' as const,
@@ -47,7 +47,7 @@ export default createBillingSchema({
           interval: 'year',
           lineItems: [
             {
-              id: 'price_starter_yearly',
+              id: 'price_1THsqLKttnWb7SsFgvjsKXzs',
               name: 'Starter',
               cost: 290,
               type: 'flat' as const,
@@ -82,7 +82,7 @@ export default createBillingSchema({
           interval: 'month',
           lineItems: [
             {
-              id: 'price_pro_monthly',
+              id: 'price_1THsqLKttnWb7SsFlWPf5IdP',
               name: 'Pro',
               cost: 59,
               type: 'flat' as const,
@@ -96,7 +96,7 @@ export default createBillingSchema({
           interval: 'year',
           lineItems: [
             {
-              id: 'price_pro_yearly',
+              id: 'price_1THsqMKttnWb7SsFZq3A4QkU',
               name: 'Pro',
               cost: 590,
               type: 'flat' as const,
@@ -130,7 +130,7 @@ export default createBillingSchema({
           interval: 'month',
           lineItems: [
             {
-              id: 'price_verband_monthly',
+              id: 'price_1THsqNKttnWb7SsFGv7YskgJ',
               name: 'Verband',
               cost: 199,
               type: 'flat' as const,
@@ -144,7 +144,7 @@ export default createBillingSchema({
           interval: 'year',
           lineItems: [
             {
-              id: 'price_verband_yearly',
+              id: 'price_1THsqNKttnWb7SsFhNl2bVn8',
               name: 'Verband',
               cost: 1990,
               type: 'flat' as const,
@@ -178,7 +178,7 @@ export default createBillingSchema({
           interval: 'month',
           lineItems: [
             {
-              id: 'price_enterprise_monthly',
+              id: 'price_1THsqOKttnWb7SsFlLjfLw72',
               name: 'Enterprise',
               cost: 349,
               type: 'flat' as const,
@@ -192,7 +192,7 @@ export default createBillingSchema({
           interval: 'year',
           lineItems: [
             {
-              id: 'price_enterprise_yearly',
+              id: 'price_1THsqOKttnWb7SsF8Sr12isW',
               name: 'Enterprise',
               cost: 3490,
               type: 'flat' as const,