feat(auth,admin): add LinuxDO OAuth login and admin settings

Implement LinuxDO OAuth authorization-code login flow with state validation and local account binding, and add admin APIs/UI to manage OAuth runtime settings with URL validation and secret masking.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

settings.json

@@ -12,5 +12,14 @@
   "max_action_length": 280,
   "app_host": "0.0.0.0",
   "app_port": 8095,
-  "debug": true
+  "debug": true,
+  "linuxdo_oauth": {
+    "enabled": false,
+    "client_id": "",
+    "client_secret": "",
+    "authorize_url": "https://connect.linux.do/oauth2/authorize",
+    "token_url": "https://connect.linux.do/oauth2/token",
+    "userinfo_url": "https://connect.linux.do/api/user",
+    "scope": "read"
+  }
 }