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_group_tools/utils/media.py

@@ -0,0 +1,27 @@
+import base64
+import random
+from pathlib import Path
+
+
+def image_to_base64(file_path: Path) -> str | None:
+    """图片转 base64,文件不存在时返回 None"""
+    if not file_path.is_file():
+        return None
+    with open(file_path, "rb") as f:
+        return "base64://" + base64.b64encode(f.read()).decode()
+
+
+def audio_to_base64(file_path: Path) -> str:
+    """音频转 base64"""
+    with open(file_path, "rb") as f:
+        return "base64://" + base64.b64encode(f.read()).decode()
+
+
+def random_audio(dir_path: Path) -> str | None:
+    """随机取目录下的一个语音文件并转 base64,目录不存在或为空时返回 None"""
+    if not dir_path.is_dir():
+        return None
+    files = [p for p in dir_path.iterdir() if p.is_file()]
+    if not files:
+        return None
+    return audio_to_base64(random.choice(files))