Add request body size limit configuration

[AbelOsaretin]

What

express.json() is used with default limits. The default 100KB limit should be explicitly configured and documented.

Why

An explicit limit prevents large-payload denial-of-service attacks and makes the configuration visible.

Scope

• Set explicit limit option on express.json()
• Make configurable via BODY_SIZE_LIMIT env var
• Document the default and how to change it

Acceptance Criteria

• express.json() configured with explicit limit
• BODY_SIZE_LIMIT env var supported (default: 100kb)
• Requests exceeding limit return 413 Payload Too Large
• Documented in README

Technical Context

• src/index.ts:16 — app.use(express.json())
• Express 5 json() accepts limit option
• Default: 100kb