feat(feishu): add Feishu inbound webhook, /gh commands and card actions

- add feishu_links D1 table and link store helpers
- implement X-Lark-Signature verification, url_verification, /gh login|logout|comment|merge|close and card.action.trigger Merge/Close
- render interactive cards with clickable title link, inline links and callback buttons (no whole-card card_link)
- bind Feishu account in OAuth callback
- document event subscription and required scopes

migrations/0011_feishu_links.sql

@@ -0,0 +1,4 @@
+CREATE TABLE IF NOT EXISTS feishu_links (
+  feishu_user_id TEXT PRIMARY KEY,
+  github_user_id TEXT NOT NULL
+);