Add FastAPI backend with agent system

backend/app/models/todo.py

@@ -0,0 +1,24 @@
+import uuid
+from datetime import datetime
+from sqlalchemy import Column, String, Boolean, DateTime, Enum
+from enum import Enum as PyEnum
+from app.models.base import BaseModel
+
+
+class TodoSource(str, PyEnum):
+    AI_KANBAN = "ai_kanban"
+    AI_CHAT = "ai_chat"
+    MANUAL = "manual"
+
+
+class DailyTodo(BaseModel):
+    __tablename__ = "daily_todos"
+
+    user_id = Column(String(36), nullable=False, index=True)
+    title = Column(String(500), nullable=False)
+    is_completed = Column(Boolean, default=False, nullable=False)
+    source = Column(Enum(TodoSource), default=TodoSource.MANUAL, nullable=False)
+    source_detail = Column(String(500), nullable=True)
+    source_ref_id = Column(String(36), nullable=True)
+    todo_date = Column(String(10), nullable=False)  # YYYY-MM-DD
+    completed_at = Column(DateTime, nullable=True)