Create a backend service for accelerating Channel discovery responsiveness

[mblomdahl]

As an alternative approach to waiting for acceptance and roll-out of changes in #11 and #12 (and then wait for all the video site maintainers to actually upgrade their servers with newer PeerTube software), we can deploy a backend service that queries a PeerTube server for all channels and return the subset that matches our criteria, i.e. 1) is local and 2) includes 1 or more public video. The workaround could also be applicable for #9 and #10 in the future, if we struggle with getting enhancements adopted by the PeerTube maintainers.

Functional requirements:

1. Send a GET <owntube-backend>/api/peertube/<instance_hostname>/api/v1/video-channels?isLocal=true&hasPublicVideos=true
2. Expect a response that includes all local channels with 1 or more public video, using same data structure as the native /api/v1/video-channels API endpoint

Non-functional requirements:

• Implemented in Node 22 with NestJS and Typescript
• No robust database persistence in initial version, use a container-local SQLite database
• Channel discovery results for a host should be cached and reused between requests, i.e.
  1. in request handler; request received → emit a backend-local "check cache for instance_hostname Channels" event → return the cached results
  2. in backend-local subscriber for the "check cache for instance_hostname Channels" event; a) check when was the site last queried for channels and if it was more than 1 week ago or b) send a request and check the first page of results ordered by updatedAt (descending); then if any of a) or b) are true emit a backend-local "update cache for instance_hostname Channels" event
  3. in backend-local subscriber for the "update cache for instance_hostname Channels" event, run the Channel discovery on the target site and update the cache with new results and new cache update timestamp
• If no Channel discovery result is available in the backend-local storage, it should run the Channel discovery on the target site synchronously, cache it, and then return the results to the client
• Rate limits on the PeerTube API must be respected (dynamically, subject to rate limiting response headers)
• GitHub Actions CI/CD pipeline for deploying it to OwnTube.tv Kubernetes