Critical Fix:
- Fixed 'ModuleNotFoundError' when running with uvx
- Implemented lazy imports for all heavy dependencies
- Separated stdio and SSE mode imports

Key Changes:
- _lazy_imports(): Import data analysis libraries only when needed
- _lazy_imports_sse(): Import FastAPI/SSE libraries only in SSE mode
- _create_sse_app(): Initialize FastAPI app only for SSE mode
- main_stdio(): Pure stdio mode without FastAPI dependencies
- main_sse(): SSE HTTP server mode with full web stack

Benefits:
- uvx can now successfully run the package ✅
- Faster startup time for stdio mode
- Smaller memory footprint when only using stdio
- Compatible with supergateway and Claude Desktop ✅

Testing:
- Verified stdio mode with initialize, tools/list, and tools/call
- All 5 data analysis tools working correctly
- No module import errors

Usage:
- uvx bachai-data-analysis-mcp (stdio mode, for supergateway)
- bachai-data-analysis-mcp-sse (SSE HTTP server mode)
- python main.py (SSE mode)