Adding a notification channel

Send alerts somewhere new.

A channel is a config struct and a send function in crates/alerts, plus a form in the frontend. The server and database need no changes.

  1. Write the sender. Add a module in crates/alerts/src/ with a config struct, a validate() method, and send(config, &Alert) -> Result<(), String>. Use the shared client(), which sets the timeout, and ensure_success() to turn a non-2xx response into an error. Alert provides title(), details() and text() for the message.

  2. Register it in crates/alerts/src/lib.rs: add the module, a variant to ChannelConfig (its kebab-case name becomes the type value), and arms to validate and send.

  3. Add the form. In frontend/src/routes/(dashboard)/notifications/channel-form.svelte, add the type to channelTypes, its fields, its config in buildConfig(), and the prefill for editing. Add its label to typeLabels in notifications/+page.svelte.

  4. Test and document it. Add tests in the module, and the channel to Alerts and Alert payloads.