feat: implement all 4 ORM models (SshKey, Server, Repo, SyncLog)

- Created SshKey model with encrypted private key storage
- Created Server model with Gitea configuration and SshKey relationship
- Created Repo model with repository mirror info and Server relationship
- Created SyncLog model with sync operation logs and Repo relationship
- Updated models/__init__.py to export all models
- All models use Integer (Unix timestamp) for datetime fields
- Proper bidirectional relationships using back_populates
- Added comprehensive test suite for all models and relationships

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/server.py

@@ -0,0 +1,35 @@
+"""
+服务器 ORM 模型.
+"""
+from sqlalchemy import String, Integer, Text, Boolean, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from app.database import Base
+from typing import Optional, List
+
+
+class Server(Base):
+    """
+    Gitea 服务器模型.
+
+    存储 Gitea 服务器配置和连接信息.
+    """
+    __tablename__ = "servers"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    name: Mapped[str] = mapped_column(String(100), nullable=False, unique=True)
+    url: Mapped[str] = mapped_column(String(500), nullable=False)
+    api_token: Mapped[str] = mapped_column(Text, nullable=False)
+    ssh_key_id: Mapped[int] = mapped_column(Integer, ForeignKey("ssh_keys.id"), nullable=False)
+    sync_enabled: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    schedule_cron: Mapped[Optional[str]] = mapped_column(String(50), nullable=True)
+    local_path: Mapped[str] = mapped_column(String(500), nullable=False)
+    status: Mapped[str] = mapped_column(String(20), nullable=False, default="untested")
+    created_at: Mapped[int] = mapped_column(Integer, nullable=False)
+    updated_at: Mapped[int] = mapped_column(Integer, nullable=False)
+
+    # 关系
+    ssh_key: Mapped["SshKey"] = relationship("SshKey", back_populates="servers")
+    repos: Mapped[List["Repo"]] = relationship("Repo", back_populates="server", cascade="all, delete-orphan")
+
+    def __repr__(self) -> str:
+        return f"<Server(id={self.id}, name='{self.name}', url='{self.url}', status='{self.status}')>"