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/handlers/websocket.go

@@ -213,7 +213,7 @@ func (h *WSHandler) getInitialState(userID string) map[string]interface{} {
 
 	progressMap := make(map[string]map[string]interface{})
 	for _, item := range mediaItems {
-		progress, err := h.db.GetUniversalProgress(context.Background(), database.GetUniversalProgressParams{
+		progress, err := h.db.GetUniversalProgress(rls.WithUser(context.Background(), pgUserID), database.GetUniversalProgressParams{
 			MediaItemID: item.ID,
 			UserID:      pgUserID,
 		})