Add rate limiting to high-risk

Introduce centralized rate limiting by applying defined tiers (STRICT, STANDARD, LENIENT, MODERATE) to high-risk edge functions:
- export-user-data (STRICT, 5 req/min)
- send-contact-message (STANDARD, 20 req/min)
- validate-email-backend (LENIENT, 30 req/min)
- admin-delete-user, resend-deletion-code (MODERATE)
- additional standard targets identified (request-account-deletion, cancel-account-deletion) as per guidance

Implements:
- Wrapped handlers with withRateLimit using centralized rateLimiters
- Imported from shared rate limiter module
- Annotated with comments explaining tier rationale
- Updated three initial functions and extended coverage to admin/account management functions
- Added documentation guide for rate limiting usage

This aligns with the Rate Limiting Guide and centralizes rate limit configuration for consistency.

supabase/functions/admin-delete-user/index.ts

@@ -1,5 +1,6 @@
 import { createClient } from 'https://esm.sh/@supabase/supabase-js@2.57.4';
 import { corsHeaders } from '../_shared/cors.ts';
+import { rateLimiters, withRateLimit } from '../_shared/rateLimiter.ts';
 import { edgeLogger, startRequest, endRequest } from '../_shared/logger.ts';
 import { formatEdgeError } from '../_shared/errorFormatter.ts';
 
@@ -13,7 +14,9 @@ interface DeleteUserResponse {
   errorCode?: 'aal2_required' | 'permission_denied' | 'invalid_request' | 'deletion_failed';
 }
 
-Deno.serve(async (req) => {
+// Apply moderate rate limiting (10 req/min) for admin user deletion
+// Prevents abuse of this sensitive administrative operation
+Deno.serve(withRateLimit(async (req) => {
   if (req.method === 'OPTIONS') {
     return new Response(null, { headers: corsHeaders });
   }
@@ -556,4 +559,4 @@ Deno.serve(async (req) => {
       { status: 500, headers: { ...corsHeaders, 'Content-Type': 'application/json' } }
     );
   }
-});
+}, rateLimiters.moderate, corsHeaders));