Move AI infrastructure settings to application_settings

What does this MR do and why?

Moves 5 instance-level AI infrastructure settings from ai_settings to application_settings, as the first step of splitting the larger ai_settings org-scoping work (issue #531356):

• ai_gateway_url
• ai_gateway_timeout_seconds
• enabled_instance_verbose_ai_logs
• duo_agent_platform_service_url
• self_hosted_duo_agent_platform_service_secure

These settings are infrastructure/deployment concerns that apply to the whole instance, in some cases we are unable to find a current organizations for those.

They may do better in application_settings rather than in the (soon to be org-scoped) ai_settings table.

This MR:

• adds the 5 columns to application_settings (with limits/defaults and application_setting_columns definitions);
• adds a post-deploy migration backfilling the values from the existing ai_settings record;
• switches Gitlab::Duo::Administration::VerifySelfHostedSetup and Gitlab::Duo::Developments::Setup to read/write via Gitlab::CurrentSettings;
• updates developer documentation for application settings.

The old ai_settings columns are not dropped here; column removal will follow the standard multi-release process (planned for %19.5 via ignore_columns) in a follow-up MR.

References

• Issue: #531356

Database

• Migration: db/migrate/20260705100000_add_ai_infrastructure_columns_to_application_settings.rb
• Post-deploy backfill: db/post_migrate/20260705100100_backfill_ai_infrastructure_settings_from_ai_settings.rb

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