perf(usage): coalesce session-sync notifications and serialize sync execution

Replace per-insert usage notifications with a single notification per
sync pass, guard all sync entry points (startup backfill, 60s loop,
manual sync) behind a process-wide single-flight tokio mutex, and move
blocking file/DB work onto spawn_blocking with MissedTickBehavior::Skip.

Extend SessionSyncResult with deferredFiles/suspectedDuplicates
observability fields (saturating aggregation) and add a test seam that
counts notifications even without an injected AppHandle.

src/types/usage.ts

@@ -40,6 +40,8 @@ export interface SessionSyncResult {
   imported: number;
   skipped: number;
   filesScanned: number;
+  suspectedDuplicates: number;
+  deferredFiles: number;
   errors: string[];
 }