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-backend/index.ts

@@ -1,13 +1,9 @@
 import { serve } from "https://deno.land/std@0.168.0/http/server.ts";
 import { createClient } from 'https://esm.sh/@supabase/supabase-js@2.39.3';
+import { corsHeaders } from '../_shared/cors.ts';
 import { 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',
-};
-
 // Simple request tracking
 const startRequest = () => ({ requestId: crypto.randomUUID(), start: Date.now() });
 const endRequest = (tracking: { start: number }) => Date.now() - tracking.start;