feat: Implement Phase 2 alerting system with anomaly detection

Proactive monitoring infrastructure for server health:

**Alert Service:**
- Population drop detection (configurable % threshold)
- FPS degradation monitoring (configurable FPS threshold)
- Multi-channel notifications (Discord, Pushbullet, Email)
- Spam prevention (30-min duplicate suppression)
- Severity levels (Info, Warning, Critical)

**Database:**
- alert_config table (thresholds per license)
- alert_history table (event log with metadata)
- 90-day retention with cleanup job

**Integration:**
- Discord/Pushbullet service integration
- Notification config retrieval from public_site_config
- Ready for stats pipeline integration

Purpose: Server admins get alerted when anomalies occur
(population crashes, performance degradation). Configurable
thresholds enable proactive server management.

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

backend/src/db/mod.rs

@@ -14,3 +14,4 @@ pub mod stats;
 pub mod store;
 pub mod player_sessions;
 pub mod public;
+pub mod alerts;