feat: implement Server service layer with encrypted API tokens

Implement business logic for Gitea server management:
- create_server(): Create servers with encrypted API tokens
- list_servers(): List all servers ordered by creation time
- get_server(): Retrieve server by ID
- update_server(): Update server configuration with token re-encryption
- delete_server(): Delete servers
- get_decrypted_token(): Decrypt API tokens for operations

Features:
- API token encryption using AES-256-GCM
- Automatic local_path generation based on server name
- SSH key validation before server creation
- Name uniqueness enforcement
- Timestamp tracking (created_at, updated_at)
- Repos directory auto-creation

Tests:
- 24 comprehensive test cases covering all scenarios
- Encryption verification tests
- Edge case handling (duplicates, not found, invalid references)
- All tests passing (63/63 total)

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

backend/app/services/__init__.py

@@ -0,0 +1,9 @@
+"""
+Services module.
+
+Business logic layer for application services.
+"""
+from app.services.ssh_key_service import SshKeyService
+from app.services.server_service import ServerService
+
+__all__ = ['SshKeyService', 'ServerService']