fix(import): defensive handling for closed issues missing closed_at timestamp

Fixes GH#523: older versions of bd could close issues without setting
the closed_at timestamp. When importing such issues, validation would
fail with "closed issues must have closed_at timestamp".

This fix adds defensive handling in all issue creation/validation paths:
- If status is "closed" and closed_at is nil, set closed_at to
  max(created_at, updated_at) + 1 second
- Similarly for tombstones missing deleted_at

Applied to:
- batch_ops.go: validateBatchIssuesWithCustomStatuses (main import path)
- transaction.go: CreateIssue and CreateIssues
- queries.go: CreateIssue
- multirepo.go: upsertIssueInTx

Also adds comprehensive tests for the defensive fix.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/storage/sqlite/batch_ops.go

@@ -32,6 +32,27 @@ func validateBatchIssuesWithCustomStatuses(issues []*types.Issue, customStatuses
 			issue.UpdatedAt = now
 		}
 
+		// Defensive fix for closed_at invariant (GH#523): older versions of bd could
+		// close issues without setting closed_at. Fix by using max(created_at, updated_at) + 1s.
+		if issue.Status == types.StatusClosed && issue.ClosedAt == nil {
+			maxTime := issue.CreatedAt
+			if issue.UpdatedAt.After(maxTime) {
+				maxTime = issue.UpdatedAt
+			}
+			closedAt := maxTime.Add(time.Second)
+			issue.ClosedAt = &closedAt
+		}
+
+		// Defensive fix for deleted_at invariant: tombstones must have deleted_at
+		if issue.Status == types.StatusTombstone && issue.DeletedAt == nil {
+			maxTime := issue.CreatedAt
+			if issue.UpdatedAt.After(maxTime) {
+				maxTime = issue.UpdatedAt
+			}
+			deletedAt := maxTime.Add(time.Second)
+			issue.DeletedAt = &deletedAt
+		}
+
 		if err := issue.ValidateWithCustomStatuses(customStatuses); err != nil {
 			return fmt.Errorf("validation failed for issue %d: %w", i, err)
 		}