feat: self-service worker enrollment bound to a user account

Let a signed-in user turn their own machine into a worker without the
shared token. The coordinator already binds a JWT-authenticated
registration to owner_id as untrusted; this adds the missing pieces.

userservice: long-lived worker keys (scimesh_wk_live_*, hash-at-rest)
with create/list/revoke and a public /worker-tokens/exchange that trades
a key for a short-lived JWT carrying the owner current role/verified.

python worker: SCIMESH_WORKER_KEY + SCIMESH_USERSERVICE_URL; a token
provider exchanges the key and refreshes the JWT proactively and on 401,
so a long-running worker survives token expiry. Static bearer token path
is unchanged.

coordinator UI: an "add your machine" page that mints a key and shows a
ready-to-run command, proxying key management to the userservice; the
dashboard gains an owner-scoped "my machines" section.

docs: how to run a worker from your account, plus the untrusted/quorum/
verified trust model.

coordinator/internal/storage/postgres/ui_read_repo.go

@@ -128,6 +128,32 @@ func (r *UIReadRepo) ListWorkers(ctx context.Context, limit int) ([]domain.Worke
 	return workers, rows.Err()
 }
 
+func (r *UIReadRepo) ListWorkersByOwner(ctx context.Context, owner uuid.UUID, limit int) ([]domain.Worker, error) {
+	if limit < 1 || limit > 100 {
+		return nil, domain.ErrInvalidInput
+	}
+	sql, args, err := psql.Select(workerColumns...).From("workers").
+		Where(sq.Eq{"owner_id": owner}).
+		OrderBy("last_heartbeat_at DESC", "id DESC").Limit(uint64(limit)).ToSql()
+	if err != nil {
+		return nil, err
+	}
+	rows, err := conn(ctx, r.pool).Query(ctx, sql, args...)
+	if err != nil {
+		return nil, fmt.Errorf("list workers by owner: %w", err)
+	}
+	defer rows.Close()
+	workers := make([]domain.Worker, 0)
+	for rows.Next() {
+		worker, err := scanWorker(rows)
+		if err != nil {
+			return nil, err
+		}
+		workers = append(workers, *worker)
+	}
+	return workers, rows.Err()
+}
+
 func (r *UIReadRepo) ListArtifactsByJob(ctx context.Context, jobID uuid.UUID) ([]domain.Artifact, error) {
 	sql, args, err := psql.Select(artifactColumns...).From("artifacts").Where(sq.Eq{"job_id": jobID}).OrderBy("created_at ASC", "id ASC").ToSql()
 	if err != nil {