feat: add domain option for custom domain/subdomain routing

[simulieren]

Summary

• Adds a domain config option that restricts all server-stats routes to a specific domain or subdomain
• Uses AdonisJS's native router.group().domain() API
• Supports both fixed domains (admin.example.com) and dynamic subdomains (:tenant.example.com)

Usage

import { defineConfig } from 'adonisjs-server-stats'

export default defineConfig({
  domain: 'admin.example.com',
  // ... other config
})

Changes

• ServerStatsConfig and ResolvedServerStatsConfig gain domain?: string
• AdonisRouter type shim updated with AdonisRouteGroup interface supporting .domain() chaining
• All route registrars (stats, debug, dashboard) conditionally apply .domain() when configured
• defineConfig() passes domain through to resolved config
• 5 new tests covering config resolution and route registration with/without domain

Test plan

• defineConfig({ domain: 'admin.example.com' }) passes domain to resolved config
• defineConfig({}) leaves domain undefined (no behavior change)
• Dynamic subdomain syntax (:tenant.example.com) works
• All route groups receive .domain() when domain is set
• No .domain() called when domain is not configured
• Full build passes
• TypeScript typecheck passes

Closes #9

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