feat: Implement alert creation service with FCM notifications

- Add AlertCreationService to handle alert creation and notifications.
- Implement idempotency key support to prevent duplicate alerts.
- Create alert notification service for sending FCM messages.
- Add tests for alert creation and notification services.
- Document FCM notification flow and alert creation process.

backend/migrations/007_alert_creation_idempotency.sql

@@ -0,0 +1,10 @@
+CREATE TABLE IF NOT EXISTS alert_creation_requests (
+    idempotency_key VARCHAR(255) PRIMARY KEY,
+    request_hash CHAR(64) NOT NULL,
+    alert_id INTEGER REFERENCES alerts(id) ON DELETE SET NULL,
+    response_body JSONB,
+    created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX IF NOT EXISTS idx_alert_creation_requests_created_at
+    ON alert_creation_requests(created_at);