Feature: Scheduled Analytics Reports (HTML/PDF/JSON) with Trend Snapshots

## Problem
Analytics are currently real-time and exploratory, but there is no recurring reporting for weekly/monthly review.

## Proposal
Implement **Scheduled Reports** that generate trend summaries and export artifacts.

## Scope
- Report templates (weekly health, tool usage, token spend, session throughput)
- Scheduler (cron-like UI + server execution)
- Export formats: HTML, PDF, JSON
- Report history page with downloadable artifacts

## Contents of a Report
- Session volume trends
- Agent status distribution over time
- Top tools and failure-prone operations
- Token usage breakdown and anomalies

## Technical Notes
- Persist report definitions and run logs
- Snapshot query windows with explicit timezone handling
- Cache/report generation to avoid blocking live APIs

## Acceptance Criteria
- [ ] User can create and schedule recurring reports
- [ ] Reports are generated for configured time windows reliably
- [ ] Artifacts are downloadable from UI and API
- [ ] Failed report runs include actionable error logs
- [ ] Trend calculations are consistent with in-app analytics charts