feat: multiple sources per user with per-repository attribution (#404)

A user can connect several sources (GitHub, GitHub Enterprise, GitLab, Gitea/Forgejo) and every repository remembers which source it came from. New sources table (migration 0019) with repositories.source_id backfilled by provider and host, per-user unique indexes widened to include the source, a Sources card on the Configuration page with per-source lock confirmations, and credentials resolved from each repository's own source in discovery, import, scheduling, organization mirror, retry, recovery, push targets, cleanup and reconcile. The legacy githubConfig connection fields mirror the primary source so scripts and env setups keep working.

CHANGELOG.md

@@ -8,6 +8,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Added
+- Multi-source support: connect multiple source services per user (#375 follow-up)
+  - Sources list on the Configuration page with add, edit and remove per source
+  - Per-repository source attribution, with credentials resolved from each repository's own source
+  - Per-source locking, discovery, auto import and cleanup
 - Git LFS (Large File Storage) support for mirroring (#74)
   - New UI checkbox "Mirror LFS" in Mirror Options
   - Automatic LFS object transfer when enabled