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-tauri/src/services/session_usage_codex.rs

@@ -307,6 +307,8 @@ pub fn sync_codex_usage(db: &Database) -> Result<SessionSyncResult, AppError> {
         skipped: 0,
         files_scanned: files.len() as u32,
         errors: vec![],
+        suspected_duplicates: 0,
+        deferred_files: 0,
     };
 
     if files.is_empty() {