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/routes/billing.js

@@ -45,7 +45,7 @@ router.get("/success", async (req, res) => {
     }
     // Prevent duplicate provisioning from same session
     if (provisionedSessions.has(sessionId)) {
-        res.status(409).send("This checkout session has already been used to provision a key. If you lost your key, use the key recovery feature.");
+        res.status(409).json({ error: "This checkout session has already been used to provision a key. If you lost your key, use the key recovery feature." });
         return;
     }
     try {