fix(billing): add rate limiting, body size check, and logging to checkout endpoint (BUG-079)

- Rate limit /checkout to 3 requests per IP per hour via express-rate-limit
- Reject request bodies >1KB (413)
- Log checkout session creation with client IP
- Bump version to 0.3.4

src/routes/billing.ts

@@ -1,4 +1,5 @@
 import { Router, Request, Response } from "express";
+import rateLimit from "express-rate-limit";
 import Stripe from "stripe";
 import { createProKey, downgradeByCustomer, updateEmailByCustomer } from "../services/keys.js";
 import logger from "../services/logger.js";
@@ -45,8 +46,25 @@ async function isDocFastSubscription(subscriptionId: string): Promise<boolean> {
   }
 }
 
+// Rate limit checkout: max 3 requests per IP per hour
+const checkoutLimiter = rateLimit({
+  windowMs: 60 * 60 * 1000, // 1 hour
+  max: 3,
+  keyGenerator: (req: Request) => req.ip || req.socket.remoteAddress || "unknown",
+  standardHeaders: true,
+  legacyHeaders: false,
+  message: { error: "Too many checkout requests. Please try again later." },
+});
+
 // Create a Stripe Checkout session for Pro subscription
-router.post("/checkout", async (_req: Request, res: Response) => {
+router.post("/checkout", checkoutLimiter, async (req: Request, res: Response) => {
+  // Reject suspiciously large request bodies (>1KB)
+  const contentLength = parseInt(req.headers["content-length"] || "0", 10);
+  if (contentLength > 1024) {
+    res.status(413).json({ error: "Request body too large" });
+    return;
+  }
+
   try {
     const priceId = await getOrCreateProPrice();
 
@@ -58,6 +76,9 @@ router.post("/checkout", async (_req: Request, res: Response) => {
       cancel_url: `${process.env.BASE_URL || "https://docfast.dev"}/#pricing`,
     });
 
+    const clientIp = req.ip || req.socket.remoteAddress || "unknown";
+    logger.info({ clientIp, sessionId: session.id }, "Checkout session created");
+
     res.json({ url: session.url });
   } catch (err: any) {
     logger.error({ err }, "Checkout error");