feat: persist per-tab token/cache usage across reload

Persist usage as invisible type:"usage" chunk rows (side channel):
- core: add "usage" ChunkType + UsageData; exclude usage rows from
  getChunksForTab/getTotalChunkCount; add getUsageStatsForTab aggregate
  (exported from barrel); defensive skip in groupRowsToMessages.
- api: agent-manager accumulates per-attempt usageRows and flushes them in
  the same atomic appendChunks call as the turn's content (discarded on a
  superseded fallback attempt). GET /tabs enriches rows with usageStats.
- frontend: hydrateFromBackend seeds cacheStats from usageStats (reload only;
  no re-seed on statuses reconnect, so no double-count with live events).

Tests: core DB-backed usage persistence/aggregate; api usage-row-per-event +
fallback discard; routes GET /tabs usageStats; frontend hydrate seed +
no-double-count + live-accumulation-after-seed. 495 -> 509 passing.

1 files changed, 5 insertions, 1 deletions

diff --git a/packages/api/src/routes/tabs.ts b/packages/api/src/routes/tabs.ts
index b1e9659..f52ee99 100644
--- a/packages/api/src/routes/tabs.ts
+++ b/packages/api/src/routes/tabs.ts
@@ -6,6 +6,7 @@ import {
 	getSetting,
 	getTab,
 	getTotalChunkCount,
+	getUsageStatsForTab,
 	groupRowsToMessages,
 	listOpenTabs,
 	setSetting,
@@ -27,7 +28,10 @@ export function setTabsAgentManager(
 }
 
 tabsRoutes.get("/", (c) => {
-	const tabs = listOpenTabs();
+	// Enrich each tab with its persisted usage aggregate so the frontend can
+	// seed `cacheStats` on reload without an extra round-trip. N small indexed
+	// queries — fine for tab counts.
+	const tabs = listOpenTabs().map((t) => ({ ...t, usageStats: getUsageStatsForTab(t.id) }));
 	return c.json({ tabs });
 });