feat: implement SSH Key service layer with encryption and business logic

Implemented SshKeyService class following TDD principles with comprehensive test coverage:

Service Methods:
- create_ssh_key(name, private_key, password) - Creates SSH key with AES-256-GCM encryption
- list_ssh_keys() - Lists all SSH keys (without decrypted keys)
- get_ssh_key(key_id) - Retrieves SSH key by ID
- delete_ssh_key(key_id) - Deletes key with usage validation
- get_decrypted_key(key_id) - Returns decrypted private key for Git operations

Features:
- Encrypts SSH private keys before storing using app.security.encrypt_data
- Generates SHA256 fingerprints for key identification
- Validates SSH key format (RSA, OpenSSH, DSA, EC, ED25519, PGP)
- Prevents deletion of keys in use by servers
- Base64-encoding for encrypted data storage in Text columns
- Uses app.config.settings.encrypt_key for encryption

Tests:
- 16 comprehensive test cases covering all service methods
- All tests passing (16/16)
- Tests for encryption/decryption, validation, usage checks, edge cases

Files:
- backend/app/services/ssh_key_service.py - SshKeyService implementation
- backend/tests/test_services/test_ssh_key_service.py - Test suite
- backend/tests/conftest.py - Fixed test encryption key length (32 bytes)

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

backend/tests/conftest.py

@@ -41,12 +41,16 @@ def db_session(db_engine):
 def test_encrypt_key():
     """测试加密密钥."""
     import base64
-    return base64.b64encode(b'test-key-32-bytes-long-1234567890').decode()
+    return base64.b64encode(b'test-key-32-bytes-long-123456789').decode()
 
 
 @pytest.fixture(scope="function")
 def test_env_vars(db_path, test_encrypt_key, monkeypatch):
     """设置测试环境变量."""
+    # Clear global settings to ensure fresh config
+    import app.config
+    app.config._settings = None
+
     monkeypatch.setenv("GM_ENCRYPT_KEY", test_encrypt_key)
     monkeypatch.setenv("GM_API_TOKEN", "test-token")
     monkeypatch.setenv("GM_DATA_DIR", str(db_path.parent))