feat: implement comprehensive configuration management system with multi-mirror support

- Add ServerSettings class with pydantic-settings for type-safe configuration
- Support multiple PyPI mirror sources with priority-based fallback mechanism
- Implement RepositoryConfig and RepositoryManager for multi-repository support
- Add environment variable support for all configuration options
- Include private repository authentication configuration
- Add advanced dependency analysis settings (max depth, concurrency, security)
- Provide secure credential management with sensitive data masking
- Update documentation and configuration examples
- Add comprehensive test suite with 23 test cases covering all features
- Include demo script showcasing multi-mirror configuration capabilities

Configuration features:
- Primary, additional, and fallback index URLs
- Automatic duplicate URL removal with priority preservation
- Runtime configuration reloading
- Integration with repository manager for seamless multi-source queries

Signed-off-by: longhao <hal.long@outlook.com>

examples/cursor_mcp.json

@@ -4,8 +4,14 @@
       "command": "uvx",
       "args": ["--from", "pypi-query-mcp-server", "pypi-query-mcp"],
       "env": {
-        "PYPI_INDEX_URL": "https://pypi.org/simple/",
-        "CACHE_TTL": "3600"
+        "PYPI_INDEX_URL": "https://pypi.org/pypi",
+        "PYPI_INDEX_URLS": "https://mirrors.aliyun.com/pypi/simple/,https://pypi.tuna.tsinghua.edu.cn/simple/",
+        "PYPI_EXTRA_INDEX_URLS": "https://test.pypi.org/simple/",
+        "PYPI_CACHE_TTL": "3600",
+        "PYPI_LOG_LEVEL": "INFO",
+        "PYPI_REQUEST_TIMEOUT": "30.0",
+        "PYPI_DEPENDENCY_MAX_DEPTH": "5",
+        "PYPI_DEPENDENCY_MAX_CONCURRENT": "10"
       }
     }
   }