fix(media): validate upload limit config

[rissrice2105-agent]

Fixes #91.

What changed

• parse MEDIA_MAX_BYTES as a strict positive decimal integer
• reject zero, signed, fractional, scientific, non-finite, and unsafe values
• retain the existing 100 MB fallback so malformed deployment config cannot disable the upload cap
• add focused coverage for valid and unsafe values

Validation

• node --test tests/media-upload-type.test.mjs (5/5)
• git diff --check