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]>

users.json.example

@@ -0,0 +1,14 @@
+{
+  "abc123secrettoken": {
+    "nextcloudUrl": "https://nextcloud.example.com",
+    "username": "alice",
+    "password": "alice-app-password",
+    "folder": "AudioPen"
+  },
+  "xyz789anothertoken": {
+    "nextcloudUrl": "https://nextcloud.example.com",
+    "username": "bob",
+    "password": "bob-app-password",
+    "folder": "AudioPen"
+  }
+}