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/sync_log.py

@@ -0,0 +1,31 @@
+"""
+同步日志 ORM 模型.
+"""
+from sqlalchemy import String, Integer, Text, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from app.database import Base
+from typing import Optional, List
+
+
+class SyncLog(Base):
+    """
+    同步日志模型.
+
+    记录仓库同步操作的详细信息.
+    """
+    __tablename__ = "sync_logs"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    repo_id: Mapped[int] = mapped_column(Integer, ForeignKey("repos.id"), nullable=False)
+    status: Mapped[str] = mapped_column(String(20), nullable=False)
+    started_at: Mapped[int] = mapped_column(Integer, nullable=False)
+    finished_at: Mapped[int] = mapped_column(Integer, nullable=False)
+    commits_count: Mapped[Optional[int]] = mapped_column(Integer, nullable=True)
+    error_msg: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    created_at: Mapped[int] = mapped_column(Integer, nullable=False)
+
+    # 关系
+    repo: Mapped["Repo"] = relationship("Repo", back_populates="sync_logs")
+
+    def __repr__(self) -> str:
+        return f"<SyncLog(id={self.id}, repo_id={self.repo_id}, status='{self.status}', commits_count={self.commits_count})>"