Add multi-user support via token-based routing and users.json config

Each user gets a unique secret token embedded in their webhook URL
(/webhook/:token). Credentials are loaded from users.json at startup
instead of environment variables, making it easy to add or remove users
without redeploying.

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

.gitignore

@@ -1,2 +1,3 @@
 node_modules/
-.env
+.env
+users.json