将框架从 Flask 迁移到 FastAPI

主要改动：
- 更新依赖：使用 fastapi、uvicorn 替代 Flask
- 数据库层：从 Flask-SQLAlchemy 迁移到纯 SQLAlchemy
- 新增 database.py 管理数据库连接和会话
- 路由层：从 Blueprint 迁移到 APIRouter，添加 Pydantic 模型验证
- 应用层：使用 FastAPI 中间件替代 Flask 插件
- 启动方式：使用 uvicorn 替代 Flask 开发服务器
- 更新 Docker 配置以支持 FastAPI

优势：
- 更高的性能和异步支持
- 自动生成 OpenAPI 文档
- 更好的类型安全和数据验证
- 所有 API 端点保持向后兼容

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/models.py

@@ -1,18 +1,19 @@
-from flask_sqlalchemy import SQLAlchemy
+from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, func
+from sqlalchemy.ext.declarative import declarative_base
+from sqlalchemy.orm import relationship
 from datetime import datetime
-from sqlalchemy import func
 from werkzeug.security import generate_password_hash, check_password_hash
 
-db = SQLAlchemy()
+Base = declarative_base()
 
-class User(db.Model):
+class User(Base):
     """用户模型"""
     __tablename__ = 'users'
 
-    id = db.Column(db.Integer, primary_key=True)
-    username = db.Column(db.String(80), unique=True, nullable=False)
-    password_hash = db.Column(db.String(200), nullable=False)
-    created_at = db.Column(db.DateTime, default=datetime.utcnow)
+    id = Column(Integer, primary_key=True)
+    username = Column(String(80), unique=True, nullable=False)
+    password_hash = Column(String(200), nullable=False)
+    created_at = Column(DateTime, default=datetime.utcnow)
 
     def set_password(self, password):
         """设置密码（哈希加密）"""
@@ -29,23 +30,23 @@ class User(db.Model):
             'created_at': self.created_at.isoformat() if self.created_at else None
         }
 
-class Task(db.Model):
+class Task(Base):
     """任务模型"""
     __tablename__ = 'tasks'
-    
-    id = db.Column(db.Integer, primary_key=True)
-    title = db.Column(db.String(200), nullable=False)
-    description = db.Column(db.Text)
-    polished_description = db.Column(db.Text)  # AI润色后的描述
-    status = db.Column(db.String(20), default='pending')  # pending, in_progress, completed
-    created_at = db.Column(db.DateTime, default=datetime.utcnow)
-    updated_at = db.Column(db.DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
-    
+
+    id = Column(Integer, primary_key=True)
+    title = Column(String(200), nullable=False)
+    description = Column(Text)
+    polished_description = Column(Text)  # AI润色后的描述
+    status = Column(String(20), default='pending')  # pending, in_progress, completed
+    created_at = Column(DateTime, default=datetime.utcnow)
+    updated_at = Column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow)
+
     # 关联关系
-    time_records = db.relationship('TimeRecord', backref='task', lazy=True, cascade='all, delete-orphan')
-    
-    def to_dict(self):
-        return {
+    time_records = relationship('TimeRecord', back_populates='task', cascade='all, delete-orphan')
+
+    def to_dict(self, db_session=None):
+        result = {
             'id': self.id,
             'title': self.title,
             'description': self.description,
@@ -53,27 +54,34 @@ class Task(db.Model):
             'status': self.status,
             'created_at': self.created_at.isoformat() if self.created_at else None,
             'updated_at': self.updated_at.isoformat() if self.updated_at else None,
-            'total_time': self.get_total_time()
         }
-    
-    def get_total_time(self):
+        if db_session:
+            result['total_time'] = self.get_total_time(db_session)
+        else:
+            result['total_time'] = 0
+        return result
+
+    def get_total_time(self, db_session):
         """获取任务总时长（秒）"""
-        total_seconds = db.session.query(func.sum(TimeRecord.duration)).filter(
+        total_seconds = db_session.query(func.sum(TimeRecord.duration)).filter(
             TimeRecord.task_id == self.id
         ).scalar() or 0
         return int(total_seconds)
 
-class TimeRecord(db.Model):
+class TimeRecord(Base):
     """时间记录模型"""
     __tablename__ = 'time_records'
-    
-    id = db.Column(db.Integer, primary_key=True)
-    task_id = db.Column(db.Integer, db.ForeignKey('tasks.id'), nullable=False)
-    start_time = db.Column(db.DateTime, nullable=False)
-    end_time = db.Column(db.DateTime)
-    duration = db.Column(db.Integer)  # 时长（秒）
-    created_at = db.Column(db.DateTime, default=datetime.utcnow)
-    
+
+    id = Column(Integer, primary_key=True)
+    task_id = Column(Integer, ForeignKey('tasks.id'), nullable=False)
+    start_time = Column(DateTime, nullable=False)
+    end_time = Column(DateTime)
+    duration = Column(Integer)  # 时长（秒）
+    created_at = Column(DateTime, default=datetime.utcnow)
+
+    # 关联关系
+    task = relationship('Task', back_populates='time_records')
+
     def to_dict(self):
         return {
             'id': self.id,
@@ -83,7 +91,7 @@ class TimeRecord(db.Model):
             'duration': self.duration,
             'created_at': self.created_at.isoformat() if self.created_at else None
         }
-    
+
     def calculate_duration(self):
         """计算时长"""
         if self.start_time and self.end_time: