Adds a Source dropdown to the configuration card (GitHub, GitLab beta,
Gitea/Forgejo) backed by a source provider interface with three adapters.
Discovery, mirroring, recovery and cleanup go through the provider;
repositories record their source (migration 0015); clone credentials are
built per host and a repository from another host is refused at the
mirror sites. Issues, pull requests, releases and star lists stay GitHub
only. The destination card gets a Gitea/Forgejo dropdown, and both hosts
lock once repositories exist, with an explicit confirmation to change.
Closes#375. Helps #371.
Claude-Session: https://claude.ai/code/session_01Tp9pmi65a8k5jLMQFLf4JX
