fix(ingestion): pass ShutdownSignal into issue and MR pagination loops

The orchestrator already accepted a ShutdownSignal but only checked it
between phases (after all issues fetched, before discussions). The inner
loops in ingest_issues() and ingest_merge_requests() consumed entire
paginated streams without checking for cancellation.

On a large initial sync (thousands of issues/MRs), Ctrl+C could be
unresponsive for minutes while the current entity type finished draining.

Now both functions accept &ShutdownSignal and check is_cancelled() at
the top of each iteration, breaking out promptly and committing the
cursor for whatever was already processed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/ingestion/orchestrator.rs

@@ -119,7 +119,8 @@ pub async fn ingest_project_issues_with_progress(
     };
 
     emit(ProgressEvent::IssuesFetchStarted);
-    let issue_result = ingest_issues(conn, client, config, project_id, gitlab_project_id).await?;
+    let issue_result =
+        ingest_issues(conn, client, config, project_id, gitlab_project_id, signal).await?;
 
     result.issues_fetched = issue_result.fetched;
     result.issues_upserted = issue_result.upserted;
@@ -329,6 +330,7 @@ pub async fn ingest_project_merge_requests_with_progress(
         project_id,
         gitlab_project_id,
         full_sync,
+        signal,
     )
     .await?;