Add HeXi bot codebase: custom plugins, web frontends, tests

- hexi core: message handling, rate limiting, cooldown, plugin manager
- Custom plugins: BF stats, daily check-in, quotes, persona cards, etc.
- Community plugins vendored under hexi/plugins with local fixes
- Web admin frontends (learning-chat, persona-admin), unified hexi/web
- Tests for rate_limit/cooldown/memes/persona; poetry.lock

Co-Authored-By: Claude <noreply@anthropic.com>

hexi/plugins/nonebot_plugin_video_analysis/models.py

@@ -0,0 +1,10 @@
+class DouyinFetchError(Exception):
+    """抖音数据抓取自定义异常"""
+
+    pass
+
+
+class ContentFetchError(Exception):
+    """多平台内容（B站动态/小红书笔记等）抓取通用异常"""
+
+    pass