feat(notifications): add Discord user and role mentions

[arpit-ubhadiya]

Allow optional numeric Discord user/role IDs on Discord notification channels so webhooks can ping people on every message for that entry.

What is this PR about?

This PR adds optional Discord user and role mention support to Discord notification settings. Users can enter comma-separated numeric Discord IDs, and Dokploy will include those mentions in webhook messages (via content + allowed_mentions) so the right people get pinged. Mentions apply to every message for that notification entry; create separate Discord notification entries if different people should be notified for different event types.

Checklist

Before submitting this PR, please make sure that:

• You created a dedicated branch based on the canary branch.
• You have read the suggestions in the CONTRIBUTING.md file https://github.com/Dokploy/dokploy/blob/canary/CONTRIBUTING.md#pull-request
• You have tested this PR in your local instance. If you have not tested it yet, please do so before submitting. This helps avoid wasting maintainers' time reviewing code that has not been verified by you.

Issues related (if applicable)

closes #2803

Screenshots (if applicable)