feat(backend): webhooks deliver signed alerts from an outbox with retries and a test send

backend/src/utils/env.py

@@ -41,6 +41,7 @@ class ApiSettings(BaseSettings):
     host: str = "0.0.0.0"  # noqa: S104
     port: int = 8080
     static_dir: str = "static"
+    public_url: str | None = None
 
 
 class AuthSettings(BaseSettings):