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/routes/signup.ts

@@ -10,7 +10,7 @@ const router = Router();
 const signupLimiter = rateLimit({
   windowMs: 60 * 60 * 1000,
   max: 5,
-  message: { error: "Too many signup attempts. Please try again in 1 hour.", retryAfter: "1 hour" },
+  message: { error: "Too many signup attempts. Please try again in 1 hour." },
   standardHeaders: true,
   legacyHeaders: false,
 });