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/export-user-data/index.ts

@@ -1,6 +1,7 @@
 import { serve } from 'https://deno.land/std@0.168.0/http/server.ts';
 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 { sanitizeError } from '../_shared/errorSanitizer.ts';
 import { edgeLogger, startRequest, endRequest } from '../_shared/logger.ts';
 import { formatEdgeError } from '../_shared/errorFormatter.ts';
@@ -13,7 +14,9 @@ interface ExportOptions {
   format: 'json';
 }
 
-serve(async (req) => {
+// Apply strict rate limiting (5 req/min) for expensive data export operations
+// This prevents abuse and manages server load from large data exports
+serve(withRateLimit(async (req) => {
   const tracking = startRequest();
   
   // Handle CORS preflight requests
@@ -364,4 +367,4 @@ serve(async (req) => {
       }
     );
   }
-});
+}, rateLimiters.strict, corsHeaders));