fix: improve config module with lazy init and validation

Fixes from code review:

Critical:
- Replace module-level `settings = Settings()` with lazy initialization
  via `get_settings()` function to avoid import failures when env vars
  not set

Important:
- Remove unused `import os` from test_config.py
- Add tests for computed properties (db_path, ssh_keys_dir, repos_dir)
- Add field validation for encrypt_key:
  * Validates base64 format
  * Ensures decoded key is at least 32 bytes for AES-256
- Fix Python 3.8 compatibility (use Optional[Settings] instead of | union)

All tests pass (6/6).

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

backend/app/config.py

@@ -1,6 +1,7 @@
-import os
+import base64
 from pathlib import Path
-from typing import Literal
+from typing import Optional
+from pydantic import field_validator
 from pydantic_settings import BaseSettings, SettingsConfigDict
 
 
@@ -24,6 +25,22 @@ class Settings(BaseSettings):
         env_file_encoding='utf-8',
     )
 
+    @field_validator('encrypt_key')
+    @classmethod
+    def validate_encrypt_key(cls, v: str) -> str:
+        """验证 encrypt_key 是否为有效的 base64 格式且长度足够."""
+        # Check if valid base64
+        try:
+            decoded = base64.b64decode(v, validate=True)
+        except Exception:
+            raise ValueError('encrypt_key must be valid base64 string')
+
+        # Check length (AES-256 requires 32 bytes)
+        if len(decoded) < 32:
+            raise ValueError('encrypt_key must decode to at least 32 bytes for AES-256')
+
+        return v
+
     @property
     def db_path(self) -> Path:
         """SQLite 数据库路径."""
@@ -40,4 +57,13 @@ class Settings(BaseSettings):
         return self.data_dir / 'repos'
 
 
-settings = Settings()
+# Lazy initialization - settings is loaded on first access
+_settings: Optional[Settings] = None
+
+
+def get_settings() -> Settings:
+    """获取全局配置实例（懒加载）."""
+    global _settings
+    if _settings is None:
+        _settings = Settings()
+    return _settings

backend/tests/test_config.py

@@ -1,8 +1,8 @@
-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
@@ -16,6 +16,7 @@ def test_config_defaults(test_env_vars, monkeypatch):
     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
@@ -30,3 +31,68 @@ def test_config_from_env(monkeypatch):
 
     assert settings.data_dir == Path('/custom/data')
     assert settings.port == 9000
+
+
+def test_computed_properties(monkeypatch):
+    """测试计算属性（db_path, ssh_keys_dir, repos_dir）."""
+    # Set required env vars but not GM_DATA_DIR to test default
+    monkeypatch.setenv("GM_ENCRYPT_KEY", base64.b64encode(b'test-key-32-bytes-long-1234567890').decode())
+    monkeypatch.setenv("GM_API_TOKEN", "test-token")
+    monkeypatch.delenv("GM_DATA_DIR", raising=False)
+
+    from app.config import Settings
+
+    # Test with default data_dir
+    settings = Settings()
+    assert settings.db_path == Path('./data/git_manager.db')
+    assert settings.ssh_keys_dir == Path('./data/ssh_keys')
+    assert settings.repos_dir == Path('./data/repos')
+
+    # Test with custom data_dir
+    monkeypatch.setenv("GM_DATA_DIR", "/custom/data")
+    settings = Settings()
+    assert settings.db_path == Path('/custom/data/git_manager.db')
+    assert settings.ssh_keys_dir == Path('/custom/data/ssh_keys')
+    assert settings.repos_dir == Path('/custom/data/repos')
+
+
+def test_encrypt_key_validation_invalid_base64(test_env_vars, monkeypatch):
+    """测试 encrypt_key 验证：无效的 base64 字符串."""
+    from app.config import Settings
+    from pydantic import ValidationError
+
+    monkeypatch.setenv("GM_ENCRYPT_KEY", "not-valid-base64!!!")
+
+    with pytest.raises(ValidationError) as exc_info:
+        Settings()
+
+    assert 'encrypt_key' in str(exc_info.value).lower()
+    assert 'base64' in str(exc_info.value).lower()
+
+
+def test_encrypt_key_validation_too_short(test_env_vars, monkeypatch):
+    """测试 encrypt_key 验证：解码后长度不足."""
+    from app.config import Settings
+    from pydantic import ValidationError
+
+    # Only 10 bytes instead of required 32
+    short_key = base64.b64encode(b'short-key-1').decode()
+    monkeypatch.setenv("GM_ENCRYPT_KEY", short_key)
+
+    with pytest.raises(ValidationError) as exc_info:
+        Settings()
+
+    assert 'encrypt_key' in str(exc_info.value).lower()
+    assert '32' in str(exc_info.value).lower() or 'byte' in str(exc_info.value).lower()
+
+
+def test_encrypt_key_validation_valid(test_env_vars, monkeypatch):
+    """测试 encrypt_key 验证：有效的密钥."""
+    from app.config import Settings
+
+    # Valid 32-byte key
+    valid_key = base64.b64encode(b'test-key-32-bytes-long-1234567890').decode()
+    monkeypatch.setenv("GM_ENCRYPT_KEY", valid_key)
+
+    settings = Settings()
+    assert settings.encrypt_key == valid_key