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/progress.go

@@ -4,7 +4,6 @@ import (
 	"bookhoard/internal/database"
 	wsync "bookhoard/internal/sync"
 	"bookhoard/internal/utils"
-	"context"
 	"encoding/json"
 	"net/http"
 	"strconv"
@@ -46,7 +45,7 @@ func (h *Handler) GetProgressHistory(c *echo.Context) error {
 		}
 	}
 
-	history, err := h.db.GetReadingHistory(context.Background(), database.GetReadingHistoryParams{
+	history, err := h.db.GetReadingHistory(c.Request().Context(), database.GetReadingHistoryParams{
 		UserID:      pgtype.UUID{Bytes: user.ID.Bytes, Valid: true},
 		MediaItemID: pgtype.UUID{Bytes: mediaItemID, Valid: true},
 		Limit:       int32(limit),