fix: Improve API initialization with lazy Supabase client and validation

- Move Supabase/Stripe initialization inside GET/POST handlers for lazy loading
- Add validation for missing environment variables in runtime
- Improve error handling in payment intent creation
- Clean up next.config.js environment variable configuration

This fixes potential build-time failures when environment variables are not available
during static generation.

app/api/cron/reset-invitations/route.ts

@@ -14,17 +14,20 @@ import { createClient } from '@supabase/supabase-js'
  * @audit RELIABILITY: Cron job should run exactly at Monday 00:00 UTC weekly
  */
 
-const supabaseUrl = process.env.NEXT_PUBLIC_SUPABASE_URL
-const supabaseServiceKey = process.env.SUPABASE_SERVICE_ROLE_KEY
-
-if (!supabaseUrl || !supabaseServiceKey) {
-  throw new Error('Missing Supabase environment variables')
-}
-
-const supabase = createClient(supabaseUrl, supabaseServiceKey)
-
 export async function GET(request: NextRequest) {
   try {
+    const supabaseUrl = process.env.NEXT_PUBLIC_SUPABASE_URL
+    const supabaseServiceKey = process.env.SUPABASE_SERVICE_ROLE_KEY
+
+    if (!supabaseUrl || !supabaseServiceKey) {
+      return NextResponse.json(
+        { success: false, error: 'Missing Supabase environment variables' },
+        { status: 500 }
+      )
+    }
+
+    const supabase = createClient(supabaseUrl, supabaseServiceKey)
+    
     const authHeader = request.headers.get('authorization')
     
     if (!authHeader || !authHeader.startsWith('Bearer ')) {