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/repo.py

@@ -0,0 +1,33 @@
+"""
+仓库 ORM 模型.
+"""
+from sqlalchemy import String, Integer, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from app.database import Base
+from typing import Optional, List
+
+
+class Repo(Base):
+    """
+    Git 仓库镜像模型.
+
+    存储从 Gitea 服务器同步的仓库信息.
+    """
+    __tablename__ = "repos"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    server_id: Mapped[int] = mapped_column(Integer, ForeignKey("servers.id"), nullable=False)
+    name: Mapped[str] = mapped_column(String(200), nullable=False)
+    full_name: Mapped[str] = mapped_column(String(300), nullable=False)
+    clone_url: Mapped[str] = mapped_column(String(500), nullable=False)
+    local_path: Mapped[str] = mapped_column(String(500), nullable=False)
+    last_sync_at: Mapped[Optional[int]] = mapped_column(Integer, nullable=True)
+    status: Mapped[str] = mapped_column(String(20), nullable=False, default="pending")
+    created_at: Mapped[int] = mapped_column(Integer, nullable=False)
+
+    # 关系
+    server: Mapped["Server"] = relationship("Server", back_populates="repos")
+    sync_logs: Mapped[List["SyncLog"]] = relationship("SyncLog", back_populates="repo", cascade="all, delete-orphan")
+
+    def __repr__(self) -> str:
+        return f"<Repo(id={self.id}, name='{self.name}', full_name='{self.full_name}', status='{self.status}')>"