Add URL→PDF endpoint, usage tracking middleware, free tier limits

src/middleware/usage.ts

@@ -0,0 +1,68 @@
+import { Request, Response, NextFunction } from "express";
+
+interface UsageRecord {
+  count: number;
+  monthKey: string;
+}
+
+// In-memory usage tracking (replace with Redis/DB for production)
+const usage = new Map<string, UsageRecord>();
+
+const FREE_TIER_LIMIT = 50; // 50 PDFs/month for free tier
+const PRO_KEYS = new Set(
+  (process.env.PRO_KEYS || "").split(",").map((k) => k.trim()).filter(Boolean)
+);
+
+function getMonthKey(): string {
+  const d = new Date();
+  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, "0")}`;
+}
+
+export function usageMiddleware(
+  req: Request,
+  res: Response,
+  next: NextFunction
+): void {
+  const key = req.headers.authorization?.slice(7) || "unknown";
+  const monthKey = getMonthKey();
+
+  // Pro keys have no limit
+  if (PRO_KEYS.has(key)) {
+    trackUsage(key, monthKey);
+    next();
+    return;
+  }
+
+  // Free tier limit check
+  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 unlimited conversions: https://docfast.dev/pricing",
+    });
+    return;
+  }
+
+  trackUsage(key, monthKey);
+  next();
+}
+
+function trackUsage(key: string, monthKey: string): void {
+  const record = usage.get(key);
+  if (!record || record.monthKey !== monthKey) {
+    usage.set(key, { count: 1, monthKey });
+  } else {
+    record.count++;
+  }
+}
+
+export function getUsageStats(): Record<string, { count: number; month: string }> {
+  const stats: Record<string, { count: number; month: string }> = {};
+  for (const [key, record] of usage) {
+    const masked = key.slice(0, 8) + "...";
+    stats[masked] = { count: record.count, month: record.monthKey };
+  }
+  return stats;
+}