Add modulo operator to calculator API - #23

Adds modulo (%) operation to the calculator's arithmetic operations.

Changes

• Added modulo operation to operations registry in api/controller.js
• Added test coverage for modulo operation including edge cases (negative operands, floats, divisor > dividend)

Usage

GET /arithmetic?operation=modulo&operand1=42&operand2=5
// Returns: {"result": 2}

Implementation follows existing pattern used by subtract, multiply, and divide operations with implicit number coercion.

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