Replaces the Endpoints enum and checker's per-host dispatch: checker.rs now
holds only the ReleaseSource trait (latest release + API root), each host
implements it in its own module (github.rs, forgejo.rs), and source.rs is a
factory returning a Box<dyn ReleaseSource> per package. Adding a host no
longer touches existing ones, and the pipeline only sees the trait.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
