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/isolation_test.go

@@ -137,7 +137,7 @@ func TestIsolation_SyncStateAndRunsNeverLeakAcrossUsers(t *testing.T) {
 		t.Fatalf("userA's UpdateSyncState leaked into userB's sync_state: %+v", stateB)
 	}
 
-	runID, err := db.StartSyncRun(ctx, userA, SyncKindBackfill)
+	runID, err := db.StartSyncRun(ctx, userA, SyncKindFull)
 	if err != nil {
 		t.Fatalf("StartSyncRun(a): %v", err)
 	}