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/db/schema.ts

@@ -148,6 +148,8 @@ export const indexingJobs = sqliteTable('indexing_jobs', {
 	progress: integer('progress').default(0), // 0–100
 	totalFiles: integer('total_files').default(0),
 	processedFiles: integer('processed_files').default(0),
+	stage: text('stage', { enum: ['queued', 'differential', 'crawling', 'cloning', 'parsing', 'storing', 'embedding', 'done', 'failed'] }).notNull().default('queued'),
+	stageDetail: text('stage_detail'),
 	error: text('error'),
 	startedAt: integer('started_at', { mode: 'timestamp' }),
 	completedAt: integer('completed_at', { mode: 'timestamp' }),