Add standard rate limit headers to PDF conversion endpoints

- Modified checkRateLimit to return RateLimitResult object with limit, remaining, and resetTime
- Added X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset headers to ALL responses
- Added Retry-After header to 429 responses
- Headers now provide developers visibility into their quota usage

src/middleware/pdfRateLimit.ts

@@ -43,7 +43,7 @@ function getRateLimit(apiKey: string): number {
   return isProKey(apiKey) ? PRO_RATE_LIMIT : FREE_RATE_LIMIT;
 }
 
-function checkRateLimit(apiKey: string): boolean {
+function checkRateLimit(apiKey: string): RateLimitResult {
   cleanupExpiredEntries();
   
   const now = Date.now();
@@ -51,19 +51,35 @@ function checkRateLimit(apiKey: string): boolean {
   const entry = rateLimitStore.get(apiKey);
   
   if (!entry || now >= entry.resetTime) {
+    const resetTime = now + RATE_WINDOW_MS;
     rateLimitStore.set(apiKey, {
       count: 1,
-      resetTime: now + RATE_WINDOW_MS
+      resetTime
     });
-    return true;
+    return {
+      allowed: true,
+      limit,
+      remaining: limit - 1,
+      resetTime
+    };
   }
   
   if (entry.count >= limit) {
-    return false;
+    return {
+      allowed: false,
+      limit,
+      remaining: 0,
+      resetTime: entry.resetTime
+    };
   }
   
   entry.count++;
-  return true;
+  return {
+    allowed: true,
+    limit,
+    remaining: limit - entry.count,
+    resetTime: entry.resetTime
+  };
 }
 
 function getQueuedCountForKey(apiKey: string): number {
@@ -106,10 +122,18 @@ export function pdfRateLimitMiddleware(req: Request & { apiKeyInfo?: any }, res:
   const apiKey = keyInfo?.key || "unknown";
   
   // Check rate limit first
-  if (!checkRateLimit(apiKey)) {
-    const limit = getRateLimit(apiKey);
+  const rateLimitResult = checkRateLimit(apiKey);
+  
+  // Set rate limit headers on ALL responses
+  res.set('X-RateLimit-Limit', String(rateLimitResult.limit));
+  res.set('X-RateLimit-Remaining', String(rateLimitResult.remaining));
+  res.set('X-RateLimit-Reset', String(Math.ceil(rateLimitResult.resetTime / 1000)));
+  
+  if (!rateLimitResult.allowed) {
     const tier = isProKey(apiKey) ? "pro" : "free";
-    res.status(429).json({ error: `Rate limit exceeded: ${limit} PDFs/min allowed for ${tier} tier. Retry after 60s.` });
+    const retryAfterSeconds = Math.ceil((rateLimitResult.resetTime - Date.now()) / 1000);
+    res.set('Retry-After', String(retryAfterSeconds));
+    res.status(429).json({ error: `Rate limit exceeded: ${rateLimitResult.limit} PDFs/min allowed for ${tier} tier. Retry after ${retryAfterSeconds}s.` });
     return;
   }