feat: init AI email summarization bot

- Multi-account IMAP email polling with UID tracking
- DeepSeek API integration with JSON Mode structured output
- Telegram notification with formatted MarkdownV2 message
- YAML config with dataclass-based type validation
- Graceful shutdown on SIGINT/SIGTERM
- 60s default polling interval

src/ai_client.py

@@ -0,0 +1,48 @@
+import json
+from typing import Any
+import requests
+from src.config import AIConfig
+
+SYSTEM_PROMPT = """你是一个邮件摘要助手。请分析邮件内容并以 JSON 格式返回结构化摘要。
+
+返回格式必须严格遵循以下 JSON schema：
+{
+  "subject": "邮件主题",
+  "sender": "发件人",
+  "summary": "50字以内的核心摘要",
+  "priority": "high/medium/low",
+  "action_required": true/false,
+  "action_items": ["待办事项1", "待办事项2"],
+  "key_points": ["关键要点1", "关键要点2"]
+}
+
+只返回 JSON，不要包含任何其他文字。"""
+
+
+def summarize_email(ai_cfg: AIConfig, subject: str, sender: str, body: str) -> dict[str, Any]:
+    content = f"发件人: {sender}\n主题: {subject}\n正文:\n{body[:4000]}"
+
+    payload = {
+        "model": ai_cfg.model,
+        "messages": [
+            {"role": "system", "content": SYSTEM_PROMPT},
+            {"role": "user", "content": content},
+        ],
+        "response_format": {"type": "json_object"},
+    }
+
+    headers = {
+        "Authorization": f"Bearer {ai_cfg.api_key}",
+        "Content-Type": "application/json",
+    }
+
+    resp = requests.post(
+        f"{ai_cfg.base_url.rstrip('/')}/chat/completions",
+        headers=headers,
+        json=payload,
+        timeout=60,
+    )
+    resp.raise_for_status()
+    result = resp.json()
+    raw = result["choices"][0]["message"]["content"]
+    return json.loads(raw)