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.

dist/index.js

@@ -222,12 +222,7 @@ app.use((req, res) => {
     const isApiRequest = req.path.startsWith('/v1/') || req.path.startsWith('/api') || req.path.startsWith('/health');
     if (isApiRequest) {
         // JSON 404 for API paths
-        res.status(404).json({
-            error: "Not Found",
-            message: `The requested endpoint ${req.method} ${req.path} does not exist`,
-            statusCode: 404,
-            timestamp: new Date().toISOString()
-        });
+        res.status(404).json({ error: `Not Found: ${req.method} ${req.path}` });
     }
     else {
         // HTML 404 for browser paths