fix: audit #18 rate limit cleanup (.unref), audit #25 consistent error shapes

Audit #18 - Rate limit store memory growth:
- rateLimitStore already had cleanup via cleanupExpiredEntries() per-request + 60s interval
- Added .unref() to the setInterval timer for clean graceful shutdown behaviour

Audit #25 - Consistent error response shapes:
- billing.ts: Fixed 409 plain-text response -> JSON { error: "..." }
- index.ts: Simplified 404 from 4-field object to { error: "Not Found: METHOD path" }
- signup.ts: Removed extra retryAfter field from rate-limit message object
- pdfRateLimit.ts: Merged limit/tier/retryAfter into single error message string
- usage.ts: Merged limit/used/upgrade fields into single error message string
- convert.ts: Merged detail field into error message (3 occurrences)

All error responses now consistently use {"error": "message"} shape.

src/middleware/pdfRateLimit.ts

@@ -102,12 +102,7 @@ export function pdfRateLimitMiddleware(req: Request & { apiKeyInfo?: any }, res:
   if (!checkRateLimit(apiKey)) {
     const limit = getRateLimit(apiKey);
     const tier = isProKey(apiKey) ? "pro" : "free";
-    res.status(429).json({
-      error: "Rate limit exceeded",
-      limit: `${limit} PDFs per minute`,
-      tier,
-      retryAfter: "60 seconds"
-    });
+    res.status(429).json({ error: `Rate limit exceeded: ${limit} PDFs/min allowed for ${tier} tier. Retry after 60s.` });
     return;
   }
   
@@ -128,4 +123,4 @@ export function getConcurrencyStats() {
 }
 
 // Proactive cleanup every 60s
-setInterval(cleanupExpiredEntries, 60_000);
+setInterval(cleanupExpiredEntries, 60_000).unref();

src/middleware/usage.ts

@@ -92,11 +92,7 @@ export function usageMiddleware(req: any, res: any, next: any): void {
   if (isProKey(key)) {
     const record = usage.get(key);
     if (record && record.monthKey === monthKey && record.count >= PRO_TIER_LIMIT) {
-      res.status(429).json({
-        error: "Pro tier limit reached (5,000/month). Contact support for higher limits.",
-        limit: PRO_TIER_LIMIT,
-        used: record.count,
-      });
+      res.status(429).json({ error: "Pro tier limit reached (5,000/month). Contact support for higher limits." });
       return;
     }
     trackUsage(key, monthKey);
@@ -106,12 +102,7 @@ export function usageMiddleware(req: any, res: any, next: any): void {
 
   const record = usage.get(key);
   if (record && record.monthKey === monthKey && record.count >= FREE_TIER_LIMIT) {
-    res.status(429).json({
-      error: "Free tier limit reached",
-      limit: FREE_TIER_LIMIT,
-      used: record.count,
-      upgrade: "Upgrade to Pro for 5,000 PDFs/month: https://docfast.dev/pricing",
-    });
+    res.status(429).json({ error: "Free tier limit reached (100/month). Upgrade to Pro at https://docfast.dev/#pricing for 5,000 PDFs/month." });
     return;
   }