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  superseded fallback attempt).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, 1 insertions, 0 deletions

diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 327b0a5..47f9218 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -37,6 +37,7 @@ export {
 	getChunksForTab,
 	getMessagesForTab,
 	getTotalChunkCount,
+	getUsageStatsForTab,
 	groupRowsToMessages,
 	type MessageRow,
 } from "./db/chunks.js";