Centralize CORS configuration

Consolidate CORS handling by introducing a shared supabase/functions/_shared/cors.ts and migrate edge functions to import from it. Remove inline cors.ts usage across functions, standardize headers (including traceparent and x-request-id), and prepare for environment-aware origins.

supabase/functions/validate-email/index.ts

@@ -1,12 +1,8 @@
 import { serve } from "https://deno.land/std@0.190.0/http/server.ts";
+import { corsHeaders } from '../_shared/cors.ts';
 import { startRequest, endRequest, edgeLogger } from "../_shared/logger.ts";
 import { formatEdgeError } from "../_shared/errorFormatter.ts";
 
-const corsHeaders = {
-  'Access-Control-Allow-Origin': '*',
-  'Access-Control-Allow-Headers': 'authorization, x-client-info, apikey, content-type, x-request-id',
-};
-
 // Comprehensive list of disposable email domains
 const DISPOSABLE_DOMAINS = new Set([
   // Popular disposable email services