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/migrations/008_alert_system.sql

@@ -0,0 +1,59 @@
+-- Alert configuration and history tables for Phase 2 anomaly detection
+
+-- Alert configuration per license (threshold settings)
+CREATE TABLE IF NOT EXISTS alert_config (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    license_id UUID NOT NULL REFERENCES licenses(id) ON DELETE CASCADE,
+
+    -- Population monitoring
+    population_drop_enabled BOOLEAN DEFAULT true,
+    population_drop_threshold_percent INTEGER DEFAULT 30,  -- Alert if player count drops >30% in 1 hour
+
+    -- Performance monitoring
+    fps_degradation_enabled BOOLEAN DEFAULT true,
+    fps_threshold INTEGER DEFAULT 30,  -- Alert if FPS below 30
+
+    -- Notification channels
+    notify_discord BOOLEAN DEFAULT true,
+    notify_pushbullet BOOLEAN DEFAULT false,
+    notify_email BOOLEAN DEFAULT false,
+
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    updated_at TIMESTAMPTZ DEFAULT NOW(),
+
+    CONSTRAINT fk_alert_config_license FOREIGN KEY (license_id) REFERENCES licenses(id)
+);
+
+CREATE UNIQUE INDEX idx_alert_config_license ON alert_config(license_id);
+
+-- Alert history log
+CREATE TABLE IF NOT EXISTS alert_history (
+    id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+    license_id UUID NOT NULL REFERENCES licenses(id) ON DELETE CASCADE,
+    alert_type VARCHAR(50) NOT NULL,  -- 'population_drop', 'fps_degradation', 'crash', 'wipe_failed'
+    severity VARCHAR(20) NOT NULL,    -- 'info', 'warning', 'critical'
+    title VARCHAR(255) NOT NULL,
+    message TEXT NOT NULL,
+    metadata JSONB,  -- Additional context (current_fps, player_count_before, player_count_after, etc.)
+
+    -- Notification status
+    notified_discord BOOLEAN DEFAULT false,
+    notified_pushbullet BOOLEAN DEFAULT false,
+    notified_email BOOLEAN DEFAULT false,
+
+    triggered_at TIMESTAMPTZ DEFAULT NOW(),
+
+    CONSTRAINT fk_alert_history_license FOREIGN KEY (license_id) REFERENCES licenses(id)
+);
+
+CREATE INDEX idx_alert_history_license ON alert_history(license_id);
+CREATE INDEX idx_alert_history_triggered ON alert_history(triggered_at DESC);
+CREATE INDEX idx_alert_history_type ON alert_history(alert_type);
+
+-- Insert default alert config for existing licenses
+INSERT INTO alert_config (license_id)
+SELECT id FROM licenses
+ON CONFLICT (license_id) DO NOTHING;
+
+COMMENT ON TABLE alert_config IS 'Phase 2 alert configuration per license with threshold settings';
+COMMENT ON TABLE alert_history IS 'Alert event log for anomaly detection and notifications';