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

.gitignore

@@ -2,3 +2,4 @@ node_modules/
 dist/
 .env
 *.log
+data/