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/send-contact-message/index.ts

@@ -1,6 +1,7 @@
 import { serve } from "https://deno.land/std@0.190.0/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 { edgeLogger } from "../_shared/logger.ts";
 import { createErrorResponse } from "../_shared/errorSanitizer.ts";
 import { formatEdgeError } from "../_shared/errorFormatter.ts";
@@ -338,4 +339,6 @@ The ThrillWiki Team`,
   }
 };
 
-serve(handler);
+// Apply standard rate limiting (20 req/min) for contact form submissions
+// Balances legitimate user needs with spam prevention
+serve(withRateLimit(handler, rateLimiters.standard, corsHeaders));