docs(store): update stale SyncRun doc comment

Every run recorded today is SyncKindFull (the single-pass "Sync now"
action) -- "backfill"/"incremental" only ever appear in historical
rows predating that consolidation.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/internal/store/syncruns.go

@@ -18,7 +18,10 @@ const (
 	SyncStatusError   = "error"
 )
 
-// SyncRun records one backfill or incremental sync attempt.
+// SyncRun records one sync attempt. Every run recorded today is
+// SyncKindFull (the "Sync now" action, covering backfill + incremental sync
+// + detail/workout fill in a single pass) -- "backfill"/"incremental" only
+// ever appear in historical rows predating that consolidation.
 type SyncRun struct {
 	ID                int64
 	Kind              string