feat: add configuration management

- Add Settings class using pydantic-settings
- Load config from environment variables with GM_ prefix
- Support encrypt_key and api_token (required, no defaults for security)
- Provide defaults for data_dir, host, port
- Add computed properties for db_path, ssh_keys_dir, repos_dir
- Add tests for config defaults and environment variable overrides
- Add Base class to app.models to unblock conftest.py imports

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

backend/tests/test_config.py

@@ -0,0 +1,32 @@
+import os
+import pytest
+import base64
+from pathlib import Path
+
+def test_config_defaults(test_env_vars, monkeypatch):
+    """测试配置默认值."""
+    # Clear GM_DATA_DIR to test default value
+    monkeypatch.delenv("GM_DATA_DIR", raising=False)
+
+    # Reload config to pick up the change
+    from app.config import Settings
+    settings = Settings()
+
+    assert settings.data_dir == Path('./data')
+    assert settings.host == '0.0.0.0'
+    assert settings.port == 8000
+
+def test_config_from_env(monkeypatch):
+    """测试从环境变量读取配置."""
+    # Set required security fields
+    monkeypatch.setenv("GM_ENCRYPT_KEY", base64.b64encode(b'test-key-32-bytes-long-1234567890').decode())
+    monkeypatch.setenv("GM_API_TOKEN", "test-token")
+    monkeypatch.setenv("GM_DATA_DIR", "/custom/data")
+    monkeypatch.setenv("GM_PORT", "9000")
+
+    # 重新加载配置
+    from app.config import Settings
+    settings = Settings()
+
+    assert settings.data_dir == Path('/custom/data')
+    assert settings.port == 9000