Adds self-hosted Git provider support (Forgejo, Gitea, GitLab) alongside GitHub:
per-workspace token connection, a provider-dispatched webhook, PR/MR and CI
mirroring, and the shared issue auto-link / auto-close machinery. Off until
MULTICA_VCS_SECRET_KEY is set, so existing deployments are unaffected.
Co-authored-by: Bohan <bohan@devv.ai>
