feat: add global request body size limit middleware with 413 handling

[whitezaddy]

closes #60

Branch Description

Implemented a global request body size limiter for the StellarKit API to protect against oversized JSON payload attacks.

Key Changes

• Added bodySizeLimit.js

  • Uses express.json({ limit })
  • Configurable via MAX_BODY_SIZE
  • Defaults to 10kb

• Updated index.js

  • Registered bodySizeLimit middleware globally before route handling

• Updated errorHandler.js

  • Returns 413 Payload Too Large for oversized request bodies
  • Uses structured JSON error with type: "PayloadTooLargeError"

• Added bodySizeLimit.test.js

  • Verifies oversized JSON payloads return 413
  • Verifies MAX_BODY_SIZE override behavior

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