feat(migrations): GraphQL fast path for GitHub migration + rate-limit resilience (#38694)

Adds a batched GraphQL path for GitHub migrations that fetches each
issue or pull request together with its comments, reviews, reactions and
timeline events instead of the REST N+1 pattern, sharply reducing
rate-limit pressure on large repositories.

- GraphQL is used for authenticated migrations with issues or pull
requests and falls back to REST automatically when it fails
- timeline events (close/reopen, lock, rename, label, milestone, merge,
branch deletion, pin, auto-merge) are imported as typed comments showing
their GitHub author
- GitBucket issue migrations no longer stop after the first page,
GitBucket sends no Link header

Fixes: https://github.com/go-gitea/gitea/issues/13243
Fixes: https://github.com/go-gitea/gitea/issues/31856
Signed-off-by: David E. Lovas <david@rexovas.com>
Co-authored-by: bircni <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: Giteabot <teabot@gitea.io>

modules/migration/comment.go

@@ -9,7 +9,6 @@ import "time"
 // Commentable can be commented upon
 type Commentable interface {
 	Reviewable
-	GetContext() DownloaderContext
 }
 
 // Comment is a standard comment information

modules/migration/review.go

@@ -8,6 +8,7 @@ import "time"
 // Reviewable can be reviewed
 type Reviewable interface {
 	GetLocalIndex() int64
+	GetContext() DownloaderContext
 
 	// GetForeignIndex presents the foreign index, which could be misused:
 	// For example, if there are 2 Gitea sites: site-A exports a dataset, then site-B imports it: