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/syncruns.go

@@ -7,12 +7,10 @@ import (
 )
 
 const (
-	SyncKindBackfill    = "backfill"
-	SyncKindIncremental = "incremental"
-	// SyncKindFull is a manually-triggered "Sync now" pass: Backfill followed
-	// by IncrementalSync followed by FillPendingDetails, recorded as one run
-	// so the reported activity count covers the whole action instead of only
-	// whichever stage happened to finish last.
+	// SyncKindFull is a manually-triggered "Sync now" pass: backfillCore
+	// followed by incrementalSyncCore followed by FillPendingDetails,
+	// recorded as one run so the reported activity count covers the whole
+	// action instead of only whichever stage happened to finish last.
 	SyncKindFull = "full"
 
 	SyncStatusRunning = "running"