refactor(sync): remove dead Backfill/IncrementalSync exported wrappers

Both were only reachable via the periodic background sync loop removed
in 4d2cbe4 -- nothing in production calls them anymore, only tests did.
FullSync already calls backfillCore/incrementalSyncCore directly.
Rewrite the affected tests to call the *Core functions (still exported
within the package) instead, dropping the now-redundant standalone
SyncRun-recording assertion covered by TestFullSync_RecordsOneCombinedSyncRun.

backend/internal/store/users_test.go

@@ -126,7 +126,7 @@ func TestDeleteUser_RemovesUserAndCascadesEverything(t *testing.T) {
 	if err := db.UpdateSyncState(ctx, userID, "2020-01-01", true); err != nil {
 		t.Fatalf("UpdateSyncState: %v", err)
 	}
-	if _, err := db.StartSyncRun(ctx, userID, SyncKindBackfill); err != nil {
+	if _, err := db.StartSyncRun(ctx, userID, SyncKindFull); err != nil {
 		t.Fatalf("StartSyncRun: %v", err)
 	}