Add per-server or per-team default build server configuration

[banjo]

Problem

The build server can only be enabled per-resource, and the Docker Image field (e.g., ghcr.io/myorg/myapp) must be manually filled in for every new application. There is no default or base URL to inherit.

This means each new app requires manually looking up the correct registry URL from an existing project in my case, because I keep forgetting it.

Proposed Solution

Add optional default build server settings at the server or team level:

• Default Build Server — all new (or all) applications on that server/team inherit the build server automatically.
• Docker Registry Base URL — set a base URL (e.g., ghcr.io/myorg/) and Coolify auto-generates the full image as {base_url}/{resource_name}.

Per-resource overrides should still be possible for apps that need different settings.