feat: add configurable notification dispatch retries (#1655)

* feat: add configurable notification dispatch retries

Allow each node to set 0-3 extra in-process delivery attempts with a fixed
one-second delay for routes, agents, and Test dispatch.

* fix: harden notification retry settings load/save and channel tests

Guard Delivery retries against failed and out-of-order same-node settings responses, and cover Slack/webhook retry classification alongside Discord.

* fix: clear Delivery retries saving state and correct screenshot alt

Separate save-request ownership from value-generation invalidation so a successful PATCH cannot leave Save retries stuck on Saving, reset saving on node switch, and align the Channels screenshot alt with the committed image.

* fix: surface invalid notification retry settings instead of false saved clamp

Align Channels GET handling with the backend strict 0-3 parser so stored values like 9 or 1.5 show as error needing repair, matching runtime fallback to 0 instead of displaying a clamped saved policy.

docs/reference/settings.mdx

@@ -380,6 +380,8 @@ Quick reference:
 
 Configure external destinations for alert notifications. Four agent types are available on separate tabs: **Discord**, **Slack**, **Webhook**, and **Apprise**. The masthead publishes a **CHANNELS** pill showing how many agents are enabled (for example, `2/4`).
 
+**Delivery retries** (admin-only) sets how many extra in-process attempts (0 to 3, default 0) this node makes after a transient channel failure, with a fixed one-second delay between attempts. There is no durable queue; ambiguous network failures can produce duplicate notifications.
+
 For each agent:
 
 | Field | Description |