mAi: #9 - imagen.series (batch tries 1-10 + selection)

Schema (applied via migration imagen_series_init):
- imagen.series parent table (prompt + params + count CHECK 1..10 + selected_image_id)
- imagen.jobs += series_id (FK) + series_idx
- imagen.images += series_id (FK)
- Owner-scoped RLS on series (SELECT/INSERT/UPDATE) + grants
- Partial indexes WHERE series_id IS NOT NULL on both child tables

Worker pipeline:
- worker.Job += SeriesID, populated from imagen.jobs.series_id via the
  claim query.
- cloud.SyncRequest += SeriesID; insertRow writes series_id when non-empty,
  omits the key when empty so solo runs leave the column NULL.
- maybeCloudSync threads seriesID from job.SeriesID through to the cloud
  sink. generate.go (CLI) always passes "" — solo path unchanged.

Tests:
- worker: SeriesID propagates from Job to fakePipeline.lastJob unchanged,
  solo job keeps it empty.
- cloud: SyncRequest.SeriesID lands as row.series_id in the POST body;
  empty SeriesID omits the key entirely.

Refs ImaGen#9.

cmd/imagen/worker.go

@@ -112,6 +112,9 @@ func (q *pgxQueue) Close() {
 // returns it. FOR UPDATE SKIP LOCKED is belt + braces against a second worker
 // process — out of scope for v1 but cheap insurance.
 func (q *pgxQueue) ClaimNextPending(ctx context.Context) (*worker.Job, error) {
+	// series_id is nullable on imagen.jobs (solo run when NULL); cast to text
+	// with COALESCE so pgx scans into a plain Go string. Empty string =
+	// solo run; the pipeline skips series propagation in that case.
 	const stmt = `
 		UPDATE imagen.jobs
 		SET status='running', started_at=now()
@@ -126,11 +129,13 @@ func (q *pgxQueue) ClaimNextPending(ctx context.Context) (*worker.Job, error) {
 		          COALESCE(model,''),
 		          COALESCE(width, 0), COALESCE(height, 0),
 		          COALESCE(steps, 0), COALESCE(seed, 0),
-		          COALESCE(style,'')`
+		          COALESCE(style,''),
+		          COALESCE(series_id::text, '')`
 	var j worker.Job
 	err := q.conn.QueryRow(ctx, stmt).Scan(
 		&j.ID, &j.OwnerUserID, &j.Prompt, &j.Backend,
 		&j.Model, &j.Width, &j.Height, &j.Steps, &j.Seed, &j.Style,
+		&j.SeriesID,
 	)
 	if errors.Is(err, pgx.ErrNoRows) {
 		return nil, nil
@@ -258,7 +263,7 @@ func (p *workerPipeline) Run(ctx context.Context, job worker.Job) worker.Outcome
 		// config, the worker can't serve flexsiebels at all.
 		return worker.Outcome{Err: fmt.Errorf("output.cloud_sync=off in config; the worker requires cloud_sync=on or auto")}
 	}
-	syncRes, syncErr := maybeCloudSync(ctx, p.cfg, false, job.OwnerUserID, paths, in, res, dimOrFallback(job.Width, res, "width"), dimOrFallback(job.Height, res, "height"))
+	syncRes, syncErr := maybeCloudSync(ctx, p.cfg, false, job.OwnerUserID, job.SeriesID, paths, in, res, dimOrFallback(job.Width, res, "width"), dimOrFallback(job.Height, res, "height"))
 	if syncErr != nil {
 		return worker.Outcome{Err: fmt.Errorf("cloud sync: %w", syncErr)}
 	}