fix(api): load repo before assignee update in bulk endpoint

Fixes nil pointer dereference in BulkSetIssueAssignees — the issue's
Repo field must be loaded before UpdateAssignees calls CanAssignTo.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

routers/api/v1/repo/issue_bulk.go

@@ -344,6 +344,12 @@ func BulkSetIssueAssignees(ctx *context.APIContext) {
 			continue
 		}
 
+		if err := issue.LoadRepo(ctx); err != nil {
+			result.Failures[index] = err.Error()
+			result.FailureCount++
+			continue
+		}
+
 		if err := issue.LoadAssignees(ctx); err != nil {
 			result.Failures[index] = err.Error()
 			result.FailureCount++