refactor: workspace secrets-core + secrets-mcp MCP SaaS

- Split library (db/crypto/service) and MCP/Web/OAuth binary
- Add deploy examples and CI/docs updates

Made-with: Cursor

crates/secrets-mcp/src/oauth/wechat.rs

@@ -0,0 +1,18 @@
+use super::{OAuthConfig, OAuthUserInfo};
+/// WeChat OAuth — not yet implemented.
+///
+/// This module is a placeholder for future WeChat Open Platform integration.
+/// When ready, implement `exchange_code` following the non-standard WeChat OAuth 2.0 flow:
+/// - Token exchange uses a GET request (not POST)
+/// - Preferred user identifier is `unionid` (cross-app), falling back to `openid`
+/// - Docs: https://developers.weixin.qq.com/doc/oplatform/Website_App/WeChat_Login/Wechat_Login.html
+use anyhow::{Result, bail};
+
+#[allow(dead_code)]
+pub async fn exchange_code(
+    _client: &reqwest::Client,
+    _config: &OAuthConfig,
+    _code: &str,
+) -> Result<OAuthUserInfo> {
+    bail!("WeChat login is not yet implemented")
+}