Centralize rate limiting config

Create shared rateLimitConfig.ts with tiers (strict, moderate, lenient, generous, per-user variants) and update edge functions to import centralized rate limiters. Replace inline rate limiter usage with new config, preserving backward compatibility. Add documentation guide for rate limiting usage.

supabase/functions/process-selective-approval/index.ts

@@ -558,5 +558,5 @@ const handler = async (req: Request) => {
   }
 };
 
-// Apply rate limiting: 10 requests per minute per IP (standard tier)
-serve(withRateLimit(handler, rateLimiters.standard, corsHeaders));
+// Apply rate limiting: 10 requests per minute per IP (moderate tier for moderation actions)
+serve(withRateLimit(handler, rateLimiters.moderate, corsHeaders));