Integration tests depend on a floating `reccaster:master` tag

[anderslindho]

The recceiver integration tests pull reccaster from a mutable tag:

# server/tests/integration/resources/docker-compose/ioc-compose.yml
image: ghcr.io/channelfinder/reccaster:master

That image is republished on every push to reccaster's default branch, so the reference resolves to different content over time.

Moving these tests off build: ../client and onto a published image was an improvement; the only question is which tag we point at. While both projects lived in the same repository they advanced together, so "latest master" was a coherent thing to test against. Now that they version independently it no longer identifies a known counterpart, which means runs are not reproducible and a merge in reccaster can show up as a red build on an unrelated recceiver PR.

Could we either:

• pin to an immutable digest and bump it in its own PR, which works today, or
• pin to a released version tag once reccaster publishes them, which depends on Git tags did not get migrated to new repository reccaster#3?

Either way it may be worth keeping a separate, non-blocking job on :master so drift still gets caught early. Happy to open the PR if there is a preference.