Configurable merge train pipeline limits (1/4)

What does this MR do and why?

This is MR 1 of 4 in a stacked series that adds configurable merge train pipeline limits, addressing gitlab-org/gitlab#374188.

MR	Branch	Base	Content
1 (this MR)	374188-1-add-max-pipelines-per-merge-train-plan-limit	master	Database migrations
2	374188-2-backend-logic-and-api	374188-1-add-max-pipelines-per-merge-train-plan-limit	Backend logic, REST API, GraphQL, tests (!228371 (merged))
3	374188-3-admin-and-project-ui	374188-2-backend-logic-and-api	Admin UI, Project UI, validations (!228380 (merged))
4	374188-4-documentation	master	Documentation (!228488 (merged))

Currently, merge trains have a hardcoded limit of 20 parallel pipelines (DEFAULT_CONCURRENCY = 20 in ee/app/services/merge_trains/refresh_service.rb). For teams with large pipelines, this overwhelms CI runners with no workaround.

This MR adds two database columns to support both iterations from the issue:

• plan_limits.max_pipelines_per_merge_train (smallint, default: 20, NOT NULL) Instance-wide limit configurable by self-managed admins via the Plan Limits API and Admin UI. (Iteration 1)

• project_ci_cd_settings.max_pipelines_per_merge_train (smallint, nullable) Project-level override. NULL means the plan limit applies. When set, the effective limit is min(plan_limit, project_limit). The plan limit always wins over the project limit. (Iteration 2)

Files to review in this MR

• db/migrate/20260330195543_add_max_pipelines_per_merge_train_to_plan_limits.rb - New migration
• db/migrate/20260330195601_add_max_pipelines_per_merge_train_to_project_ci_cd_settings.rb - New migration
• db/schema_migrations/20260330195543 - Auto-generated
• db/schema_migrations/20260330195601 - Auto-generated
• db/structure.sql - Auto-generated schema update

Database migrations

Migration 1: plan_limits

add_column :plan_limits, :max_pipelines_per_merge_train,
  :smallint, default: 20, null: false

Small table (one row per plan), safe to add column with default.

Migration 2: project_ci_cd_settings

add_column :project_ci_cd_settings, :max_pipelines_per_merge_train,
  :smallint, default: nil

Nullable column with no default, so no table rewrite required.

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.

• This MR contains only database migrations (no application logic)
• Migrations are reversible (change method)
• Column types and defaults match the implementation guide in the issue

Related issues

Part of gitlab-org/gitlab#374188 (parent epic: gitlab-org&2219)"