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/ssh_key.py

@@ -0,0 +1,28 @@
+"""
+SSH 密钥 ORM 模型.
+"""
+from sqlalchemy import String, Integer, Text
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from app.database import Base
+from typing import Optional, List
+
+
+class SshKey(Base):
+    """
+    SSH 密钥模型.
+
+    存储加密的 SSH 私钥，用于 Git 操作.
+    """
+    __tablename__ = "ssh_keys"
+
+    id: Mapped[int] = mapped_column(Integer, primary_key=True, autoincrement=True)
+    name: Mapped[str] = mapped_column(String(100), nullable=False, unique=True)
+    private_key: Mapped[str] = mapped_column(Text, nullable=False)
+    fingerprint: Mapped[Optional[str]] = mapped_column(String(64), nullable=True)
+    created_at: Mapped[int] = mapped_column(Integer, nullable=False)
+
+    # 关系
+    servers: Mapped[List["Server"]] = relationship("Server", back_populates="ssh_key")
+
+    def __repr__(self) -> str:
+        return f"<SshKey(id={self.id}, name='{self.name}', fingerprint='{self.fingerprint}')>"