Build Trigger

• Added support for configuring a separate build hook URL per site (multisite).
• Build hook URLs now support environment variables and aliases (e.g. $BUILD_HOOK_MAIN).
• The Build Trigger section now lets you pick a site and trigger its build.
• Build hook requests now accept any 2xx response (services such as Netlify and Vercel often return 201) and handle request failures gracefully.
• Existing single-URL configurations are automatically migrated to the primary site.

The buildHookUrl setting has been replaced by a per-site buildHookUrls map. Existing configurations are migrated automatically to the primary site on update.

• Initial release