Add factorial computation: utility module + REST endpoint

[Copilot]

Adds factorial calculation capability to the Task Manager API.

Changes

• factorial.js — pure utility function; validates input is a non-negative integer ≤ 18 (cap enforces Number.MAX_SAFE_INTEGER safety; factorial(19) overflows safe integer range)
• index.js — new GET /api/factorial/:n route returning { n, result }
• tests/factorial.test.js — 13 tests covering utility edge cases and HTTP responses

Usage

GET /api/factorial/5
// → { "n": 5, "result": 120 }

GET /api/factorial/19
// → 400 { "error": "Input must not exceed 18 to ensure precise integer results" }

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