feat(TRUEREF-0022): complete iteration 0 — worker-thread indexing, parallel jobs, SSE progress

- Move IndexingPipeline.run() into Worker Threads via WorkerPool
- Add dedicated embedding worker thread with single model instance
- Add stage/stageDetail columns to indexing_jobs schema
- Create ProgressBroadcaster for SSE channel management
- Add SSE endpoints: GET /api/v1/jobs/:id/stream, GET /api/v1/jobs/stream
- Replace UI polling with EventSource on repo detail and admin pages
- Add concurrency settings UI and API endpoint
- Build worker entries separately via esbuild

src/lib/server/mappers/indexing-job.mapper.ts

@@ -10,6 +10,8 @@ export class IndexingJobMapper {
 			progress: entity.progress,
 			totalFiles: entity.total_files,
 			processedFiles: entity.processed_files,
+			stage: entity.stage,
+			stageDetail: entity.stage_detail,
 			error: entity.error,
 			startedAt: entity.started_at != null ? new Date(entity.started_at * 1000) : null,
 			completedAt: entity.completed_at != null ? new Date(entity.completed_at * 1000) : null,
@@ -26,6 +28,8 @@ export class IndexingJobMapper {
 			progress: domain.progress,
 			totalFiles: domain.totalFiles,
 			processedFiles: domain.processedFiles,
+			stage: domain.stage,
+			stageDetail: domain.stageDetail,
 			error: domain.error,
 			startedAt: domain.startedAt,
 			completedAt: domain.completedAt,