fix: self-service signup, unified key store, persistent data volume

- Added /v1/signup/free endpoint for instant API key provisioning
- Built unified key store (services/keys.ts) with file-based persistence
- Refactored auth middleware to use key store (no more hardcoded env keys)
- Refactored usage middleware to check key tier from store
- Updated billing to use key store for Pro key provisioning
- Landing page: replaced mailto: link with signup modal
- Landing page: Pro checkout button now properly calls /v1/billing/checkout
- Added Docker volume for persistent key storage
- Success page now renders HTML instead of raw JSON
- Tested: signup → key → PDF generation works end-to-end

src/routes/signup.ts

@@ -0,0 +1,33 @@
+import { Router, Request, Response } from "express";
+import { createFreeKey } from "../services/keys.js";
+
+const router = Router();
+
+// Self-service free API key signup
+router.post("/free", (req: Request, res: Response) => {
+  const { email } = req.body;
+
+  if (!email || typeof email !== "string") {
+    res.status(400).json({ error: "Email is required" });
+    return;
+  }
+
+  // Basic email validation
+  if (!/^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email)) {
+    res.status(400).json({ error: "Invalid email address" });
+    return;
+  }
+
+  const keyInfo = createFreeKey(email.trim().toLowerCase());
+
+  res.json({
+    message: "Welcome to DocFast! 🚀",
+    apiKey: keyInfo.key,
+    tier: "free",
+    limit: "100 PDFs/month",
+    docs: "https://docfast.dev/#endpoints",
+    note: "Save this API key — it won't be shown again.",
+  });
+});
+
+export { router as signupRouter };