Add ai_flow_triggers table and ::Ai::FlowTrigger model

What does this MR do and why?

This MR adds the database table for AI Flow Triggers, which will be used to configure and trigger AI workflows based on specific events within GitLab projects.

Database Schema

• New table: ai_flow_triggers with the following columns:
  • project_id (bigint, not null, foreign key with cascade delete)
  • user_id (bigint, not null, indexed)
  • config_path (text, max 255 chars, nullable) - Path to AI flow configuration file
  • description (text, max 255 chars, not null) - Human-readable description
  • event_types (smallint array, default empty, not null) - Types of events that trigger the flow
  • Standard timestamps (created_at, updated_at)

Project Integration

• Added has_many :ai_flow_triggers association to the EE Project model

This is extracted from !200193 (closed) and provides the foundational data layer for the AI Flow Triggers feature.

Related issue: https://gitlab.com/gitlab-org/gitlab/-/issues/559051+