Add PUBLIC_BASE_URL and refine OAuth scopes

.env.example

@@ -16,6 +16,9 @@ OAUTH_CACHE_TTL_SECONDS=300
 # MCP server configuration
 MCP_HOST=127.0.0.1
 MCP_PORT=8080
+# Optional external URL used in OAuth metadata when running behind reverse proxies.
+# Example: PUBLIC_BASE_URL=https://gitea-mcp.hiddenden.cafe
+PUBLIC_BASE_URL=
 ALLOW_INSECURE_BIND=false
 
 # Logging / observability