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/notifications.rs

@@ -1,15 +1,40 @@
 use sqlx::PgPool;
 use uuid::Uuid;
-use anyhow::Result;
+use anyhow::{Context, Result};
+use serde::{Deserialize, Serialize};
 
-// TODO: Define NotificationConfig struct (id, server_id, discord_webhook_url, events jsonb, enabled, created_at, updated_at)
-
-/// Fetch the notification configuration for a server.
-pub async fn get_notification_config(pool: &PgPool, server_id: Uuid) -> Result<()> {
-    todo!()
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct NotificationConfig {
+    pub license_id: Uuid,
+    pub server_name: String,
+    pub discord_webhook_url: Option<String>,
+    pub pushbullet_api_key: Option<String>,
 }
 
-/// Insert or update the notification configuration for a server.
-pub async fn upsert_notification_config(pool: &PgPool, server_id: Uuid, discord_webhook_url: Option<&str>, events: &str, enabled: bool) -> Result<()> {
-    todo!()
+/// Fetch the notification configuration for a license
+pub async fn get_notification_config(pool: &PgPool, license_id: Uuid) -> Result<NotificationConfig> {
+    // Join with licenses to get server_name and public_site_config to get webhook/pushbullet
+    let row = sqlx::query!(
+        r#"
+        SELECT
+            l.id as license_id,
+            l.server_name,
+            psc.discord_webhook_url,
+            psc.pushbullet_api_key
+        FROM licenses l
+        LEFT JOIN public_site_config psc ON psc.license_id = l.id
+        WHERE l.id = $1
+        "#,
+        license_id
+    )
+    .fetch_one(pool)
+    .await
+    .context("Failed to fetch notification config")?;
+
+    Ok(NotificationConfig {
+        license_id: row.license_id,
+        server_name: row.server_name,
+        discord_webhook_url: row.discord_webhook_url,
+        pushbullet_api_key: row.pushbullet_api_key,
+    })
 }