feat: Phase 2 data aggregation pipeline (Strike 4A)

Backend:
- Stats ingestion consumer subscribing to corrosion.*.stats NATS subject
- Hourly aggregation scheduler (runs :05 past every hour)
- Daily cleanup job (03:00 UTC) with 7-day raw / 90-day hourly retention
- Analytics API (summary, timeseries, CSV export)
- Complete stats DB queries with aggregation and cleanup

Frontend:
- Analytics dashboard with ECharts integration
- Player count and server performance charts
- Time range selector (24h/7d/30d)
- CSV export functionality
- Real-time data loading

Infrastructure:
- Exposed NatsBridge.jetstream for consumer access
- Background service initialization in main.rs

Data flow: Plugin → NATS → Consumer → DB → Aggregation → API → Charts

Unblocks Strike 4B (dashboards) and 4C (alerting).

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/src/api/mod.rs

@@ -14,3 +14,4 @@ pub mod store;
 pub mod early_access;
 pub mod admin;
 pub mod ws;
+pub mod analytics;