feat(scanner): scanner entrypoints + generic jobs carry RLS identity; SeedDB harness fallback

The full suite enumerated the remaining triage class: tests drive the
scanner directly (ScanFolders(context.Background())) and read seeded
media via Queries with bare contexts — every governed read raised.

- MediaScanner.ScanFolders self-declares its configured admin
  (SetAdminID — set by main AND the test helpers): a scan is server
  maintenance, product-correct regardless of caller.
- ScopedDB gains a Fallback identity, exposed as NewSeedDB for the
  TEST HARNESS ONLY: setup.DB falls back to rls.SeedIdentity when the
  context carries none, so direct seeding/verification helpers work
  uniformly instead of per-site context threading. rls.SeedIdentity is
  the shared constant (no library_visibility rows => full visibility).
- generic job creation (jobs.go) stamps the requesting admin into the
  job Context, same as the scan endpoint.

The sync queue processor stays identity-free: sync_queue is not an
RLS-governed table.

internal/rls/rls.go

@@ -15,6 +15,7 @@ package rls
 import (
 	"context"
 
+	"github.com/google/uuid"
 	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgtype"
 )
@@ -44,6 +45,13 @@ func WithUser(ctx context.Context, uid pgtype.UUID) context.Context {
 	return context.WithValue(ctx, CtxKeyUserID, uid)
 }
 
+// SeedIdentity is the harness seeding identity (plan §5): a valid UUID
+// with NO library_visibility rows, so RLS grants it full visibility.
+// It exists so test seeding and direct DB verification can read and
+// write governed tables without impersonating a real user. Product
+// code never runs as it.
+var SeedIdentity = pgtype.UUID{Bytes: [16]byte(uuid.MustParse("00000000-0000-0000-0000-000000000002")), Valid: true}
+
 // DeclareUser sets the RLS identity GUC inside an ALREADY-OPEN
 // transaction. The `true` argument is is_local: transaction-scoped,
 // evaporating at COMMIT/ROLLBACK — NEVER session-scoped on a pooled