feat(external-upstream): add external upstream service support

[EvanSchleret]

What is this PR about?

This PR adds support for managing external upstream services in Dokploy.

It introduces:

• a new externalUpstream service type with schema, service layer, API routes, and project/environment integration
• Traefik config generation for routing domains to external upstream targets
• CIDR-based validation and settings for restricting upstream targets
• UI flows to create, update, duplicate, and inspect external upstreams
• clearer domain UX for external upstreams, including public path vs upstream path labeling

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)

N/A

Screenshots (if applicable)