feat: add alert for stack deploy failures

[pelluch]

Stacks currently have alerts for state changes, but not for deploy failures, which can arguably be more important to get notified about, particularly with automated deployments via webhooks. This PR implements that, in consistent style with existing implementations, that is:

• Addition of a failure_alert boolean which defaults to true, like the boolean present for actions and procedures
• Addition of a UI toggle on stacks to control the above
• Sending an alert when a stack deployment is not successful, and the implementation for the different Alerter types
• Generation of the different types

cargo build --verbose
cargo test --verbose
cargo fmt --all -- --check

all pass.

Local container builds, and tested locally, with Ntfy, Slack, Discord and Custom Alerter types:

A minimal stack with the following UI-backed compose was created:

services:
  boom:
    image: alpine:35 # Bad tag, will fail
    command: sleep infinity

Alerter tests:

1. Ntfy (same code path as Pushover)
   
2. Discord
   
3. Slack
   
4. Custom
   

The following cases were additionally tested:

1. Attempting a deploy after detaching a server from the stack
2. Stopping periphery, and immediately attempting a deploy after

Both triggered alerts as well.
Finally, tests were done after disabling "Send alert on failure" toggle in the Stack's UI - no failure alerts happened then.

Resolves #980