Add organization_id foreign key for ai_settings

What does this MR do and why?

Adds the organization_id sharding key to ai_settings (schema-only, no behavior change): See references for followups; this is a multi step MR set.

• Adds a nullable organization_id column.
• Adds a unique index on organization_id.
• Adds a foreign key to organizations with ON DELETE CASCADE.
• Backfills the single existing row to the default organization (post-deploy).

The column is left nullable for now. NOT NULL enforcement and per-organization lookup (replacing Ai::Setting.instance / SingletonRecord) are deferred to follow-up iterations, so ai_settings is tracked in allowed_organization_id_violations.

References

Related to #531356

• Next !243050 (merged)
• Next !243041 (merged)

How to set up and validate locally

1. Run the migrations:

   bundle exec rails db:migrate

2. Confirm the column, index, and FK exist:

   Ai::Setting.column_names.include?('organization_id') # => true

3. Confirm the existing row is backfilled to the default organization:

   Ai::Setting.instance.organization_id # => 1

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.