[api] da-admin POST /source/ should do a size check

With Helix limiting the file size of pages, images, PDFs etc. da-admin should follow the same rules and reject POST requests if they exceed the size limits.

Currently the following limits are enforced by the EDS Admin API:
* page / document size limit: 1MByte
* max image size limit 1MByte

Limits on sheet sizes or file sizes are currently documented (https://www.aem.live/docs/limits#file-size-limits) but not enforced.

Start at the da-admin API handler for POST /source/ and compare its request handling with the existing EDS Admin API limits. Review the documented file-size limits at https://www.aem.live/docs/limits#file-size-limits. Done means oversized pages, images, PDFs, sheets, and other documented files are rejected consistently.

Written by the indexing model from the issue text.