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>

config.py

@@ -22,6 +22,15 @@ DEFAULT_SETTINGS: dict[str, Any] = {
     "app_host": "0.0.0.0",
     "app_port": 8095,
     "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",
+    },
 }
 
 
@@ -49,6 +58,12 @@ def _load_settings() -> dict[str, Any]:
 
     merged = dict(DEFAULT_SETTINGS)
     merged.update(payload)
+    linuxdo_defaults = DEFAULT_SETTINGS["linuxdo_oauth"]
+    linuxdo_payload = payload.get("linuxdo_oauth", {})
+    if isinstance(linuxdo_payload, dict):
+        merged["linuxdo_oauth"] = {**linuxdo_defaults, **linuxdo_payload}
+    else:
+        merged["linuxdo_oauth"] = dict(linuxdo_defaults)
     return merged
 
 
@@ -82,6 +97,7 @@ class Config:
     APP_HOST = str(SETTINGS.get("app_host", DEFAULT_SETTINGS["app_host"]))
     APP_PORT = int(SETTINGS.get("app_port", DEFAULT_SETTINGS["app_port"]))
     DEBUG = bool(SETTINGS.get("debug", DEFAULT_SETTINGS["debug"]))
+    LINUXDO_OAUTH = SETTINGS.get("linuxdo_oauth", DEFAULT_SETTINGS["linuxdo_oauth"])
 
 
 class TestConfig(Config):