Fix post blob hydration to use proper AT Protocol API

[scarnecchia]

Problem

Post blob hydration is currently broken - it's using CDN paths instead of the proper AT Protocol API endpoint.

Current Implementation

The BlobProcessor is fetching from CDN URLs:

https://cdn.bsky.app/img/feed_thumbnail/plain/${did}/${cid}@jpeg
https://cdn.bsky.app/img/feed_fullsize/plain/${did}/${cid}@jpeg

This doesn't work reliably and requires auth context.

Solution

Apply the same fix we used for profile blobs:

1. Resolve the user's PDS endpoint from their DID document via plc.wtf
2. Use the proper AT Protocol API: ${pdsEndpoint}/xrpc/com.atproto.sync.getBlob?did=${did}&cid=${cid}

Reference

See ProfileHydrationService.processProfileBlob() for the working implementation.

Also check docs/profile-blob-hydration.md section 7 for details on proper blob fetching.

Files to Update

• src/blobs/processor.ts - Update processBlob() method
• May need to add PDS resolution logic or import from profile service