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/middleware/usage.ts

@@ -1,19 +1,13 @@
 import { Request, Response, NextFunction } from "express";
+import { isProKey } from "../services/keys.js";
 
 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 = 100; // 100 PDFs/month for free tier
-import { isProKey as isRuntimeProKey } from "../routes/billing.js";
-
-const PRO_KEYS = new Set(
-  (process.env.PRO_KEYS || "").split(",").map((k) => k.trim()).filter(Boolean)
-);
+const FREE_TIER_LIMIT = 100;
 
 function getMonthKey(): string {
   const d = new Date();
@@ -28,8 +22,8 @@ export function usageMiddleware(
   const key = req.headers.authorization?.slice(7) || "unknown";
   const monthKey = getMonthKey();
 
-  // Pro keys have no limit (env-configured or runtime-provisioned via Stripe)
-  if (PRO_KEYS.has(key) || isRuntimeProKey(key)) {
+  // Pro keys have no limit
+  if (isProKey(key)) {
     trackUsage(key, monthKey);
     next();
     return;