feat(migrations): add repository migration from bitbucket.org (#38607)

Adds migration from bitbucket.org as the Cloud variant of the Bitbucket
entry, in the web UI and via `POST /repos/migrate` with
`service=bitbucket`. It migrates the repository and pull requests with
their comments and reviews. Bitbucket Cloud has no releases, and
Atlassian removed its issues and wikis on 2026-08-20
(https://community.atlassian.com/forums/Bitbucket-articles/Announcing-sunset-of-Bitbucket-Issues-and-Wikis/ba-p/3193882).

- Private repositories need an API or access token, which also raises
the rate limit.
- Rate-limited requests back off and retry, Bitbucket sends no
`Retry-After`.
- Tests replay recorded responses, set `BITBUCKET_READ_TOKEN` to
re-record.

Co-authored-by: silverwind <me@silverwind.io>

tests/integration/repo_migrate_test.go

@@ -54,4 +54,15 @@ func TestRepoMigrate(t *testing.T) {
 		assert.Equal(t, 1, entries.Length())
 		assert.Equal(t, "Bitbucket", strings.TrimSpace(entries.Text()))
 	})
+
+	t.Run("BitbucketVariants", func(t *testing.T) {
+		for _, service := range structs.BitbucketService.Variants() {
+			resp := session.MakeRequest(t, NewRequest(t, "GET", fmt.Sprintf("/repo/migrate?service_type=%d", service)), http.StatusOK)
+			htmlDoc := NewHTMLParser(t, resp.Body)
+			assert.Equal(t, fmt.Sprint(service), htmlDoc.GetInputValueByName("service"))
+			assert.Equal(t, 2, htmlDoc.Find(".ui.compact.menu a.item").Length())
+			assert.Contains(t, htmlDoc.Find(".ui.compact.menu a.item.active").AttrOr("href", ""), fmt.Sprintf("service_type=%d&", service))
+			assert.Equal(t, service == structs.BitbucketServerService, htmlDoc.Find(`input[name="auth_username"]`).Length() == 1)
+		}
+	})
 }