fix(monitoring): persist session summaries to avoid history scans

- Maintain counts and display snapshots in source transactions with replay-safe deduplication.
- Read session lists exclusively from summaries and remove execution-only filters.
- Automatically initialize historical summaries in bounded, resumable batches during OSS startup.
- Add behavioral contracts covering accounting, retention, query cost and PostgreSQL concurrency.

Verified with the required session_summary suite and real container upgrade startup.

src/langbot/pkg/core/app.py

@@ -359,6 +359,9 @@ class Application:
         return task
 
     async def run(self):
+        from ..persistence.session_summary_backfill import initialize_application_summaries
+
+        await initialize_application_summaries(self)
         self.event_loop_monitor.start()
         from ..utils.execution_lease import maintain_execution_leases