feat(mcp_modules): add safe calculator module (no eval)

[Larslllllll]

Adds a new community MCP module calculator for issue #2 ('add more community packages').

• Secure recursive-descent expression evaluator (no eval/Function), safe against injection.
• Full operator set: + - * / % ^ with correct precedence and right-associative ^ (2^3^2 = 512). Unary +/- handled via 0-prefix (so -2^2 == -4).
• Registered in server.js under /calculator/*; controller/service; docs (README + api.md); examples; built-in self-tests.

Fixes #2