Add detailed OAuth documentation and configuration examples

- Updated `ARCHITECTURE.md` with thorough explanations of OAuth Consumer and Provider modes, supported flows, security features, and endpoints.
- Enhanced `.env.template` with environment variables for OAuth Provider mode setup.
- Expanded `README.md` to highlight OAuth Provider mode capabilities and MCP integration features.
- Added OAuth configuration section to `AGENTS.md`, including key settings for both social login and provider mode.

.env.template

@@ -40,6 +40,12 @@ OAUTH_AUTO_LINK_BY_EMAIL=true
 # OAUTH_GITHUB_CLIENT_ID=your-github-client-id
 # OAUTH_GITHUB_CLIENT_SECRET=your-github-client-secret
 
+# OAuth Provider Mode (Authorization Server for MCP/third-party clients)
+# Set OAUTH_PROVIDER_ENABLED=true to act as an OAuth 2.0 Authorization Server
+OAUTH_PROVIDER_ENABLED=false
+# IMPORTANT: Must be HTTPS in production!
+OAUTH_ISSUER=http://localhost:8000
+
 # Frontend settings
 FRONTEND_PORT=3000
 FRONTEND_URL=http://localhost:3000