Blob limits lower than Bluesky PDS

[electricduck]

While testing migration, I quickly noticed a bunch of XRPC 413: PayloadTooLarge errors indicating the blob size limits are not within parity compared to Bluesky's own PDS, and I was unable to upload large videos to my own PDS.

For some more context: https://bsky.app/profile/ducky.ws/post/3l5uvpork2g2m

[devinivy]

This can be adjusted through an environmant variable. We should ensure the distro works well for video by default, but in the meantime folks can set the following to increase the limit to 50mb:

PDS_BLOB_UPLOAD_LIMIT=52428800