feat: mirror from GitLab and Gitea/Forgejo sources (#376)

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

README.md

@@ -31,6 +31,7 @@ First user signup becomes admin. Configure GitHub and Gitea/Forgejo through the
 ## ✨ Features
 
 - 🔁 Mirror public, private, and starred GitHub repos to Gitea/Forgejo
+- 🦊 **GitLab (beta) and Gitea/Forgejo sources** - Pick the source in the configuration card. gitlab.com, Codeberg and self hosted instances mirror code, tags, wiki and LFS (issues, pull requests and releases need a GitHub source). See [docs/SOURCE_PROVIDERS.md](docs/SOURCE_PROVIDERS.md)
 - 🏛️ **GitHub Enterprise support** - Works with GHES and GHEC with data residency via `GH_API_URL`
 - 🏢 Mirror entire organizations with flexible strategies
 - 🎯 Custom destination control for repos and organizations