Add multi-agent manager support

[NightWatcher314]

Why

This adds optional multi-agent manager support for deployments that need one Nginx Proxy Manager UI to administer additional NPM instances.

The feature is intended for operators running multiple NPM nodes. A manager instance can store remote agent connection details, test them, and forward supported API operations to a selected agent. Local-only deployments continue to behave as they do today unless agents are configured.

Main changes:

• adds an agent model, migration, internal service, and routes for managing remote NPM instances
• adds an authenticated forwarding layer for supported Nginx resource APIs
• exposes agent selection on proxy host workflows that can target a remote instance
• adds an Agents page in the frontend for creating, editing, testing, enabling/disabling, and deleting remote agents
• keeps existing local API behavior as the default when no agent_id is provided

This is a non-breaking feature, but it does add new API endpoints and optional agent_id request/query handling for forwarded operations.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• API changes
• Performance improvement
• Test addition or update

Testing

• yarn lint in frontend/
• yarn build in frontend/
• yarn lint in backend/
• yarn validate-schema in backend/

AI Usage

• AI was used to write this
• AI was used to review this