Add starring support for AI catalog items (part 1 : schema)

What does this MR do and why?

Introduces the backend foundation for starring AI catalog items. This is the first in a series of MRs implementing quality signals for the AI catalog, as discussed in gitlab-org#20225 (comment 3112262413).

This MR adds:

• ai_catalog_item_stars join table storing (user_id, ai_catalog_item_id) pairs with a unique constraint
• star_count integer column on ai_catalog_items (default 0, not null, with a CHECK (star_count >= 0) constraint)
• Ai::Catalog::ItemStar model with after_create/after_destroy callbacks that update star_count in real time, mirroring the pattern used by UsersStarProject. Inactive users are excluded from count updates.
• Item#star(user), Item#unstar(user), and Item#starred_by?(user) as the primary interface

No API surface is included. A follow-up MR will add a GraphQL query field and mutation to expose starring to the frontend (WIP MR: !228667 (merged)).

Database queries

Ai::Catalog::ItemStar — star count update

Called from after_create/after_destroy callbacks on Ai::Catalog::ItemStar when a star is created or destroyed.

Query (increment):

UPDATE ai_catalog_items SET star_count = star_count + 1 WHERE id = $1

Query (decrement):

UPDATE ai_catalog_items SET star_count = star_count - 1 WHERE id = $1

Execution plan:

Update on ai_catalog_items
  ->  Index Scan using ai_catalog_items_pkey on ai_catalog_items
        Index Cond: (id = $1)

Single-row PK lookup. No full table scans.

This pattern mirrors UsersStarProject, which uses the same +1/-1 approach via update_counters. A periodic recalculation worker will be added in a follow-up to handle drift from cascade deletes (e.g. when a user is deleted).

References

• Design discussion: gitlab-org#20225 (comment 3112262413)
• Follow-up (GraphQL query + mutation): TBD
• Pattern reference: app/models/users_star_project.rb

Screenshots or screen recordings

No UI changes.

How to set up and validate locally

1. Run migrations: bin/rails db:migrate

2. In the Rails console:

   item = Ai::Catalog::Item.first
   user = User.first
   item.star(user)          # creates star, increments star_count
   item.reload.star_count   # => 1
   item.starred_by?(user)   # => true
   item.unstar(user)        # destroys star, decrements star_count
   item.reload.star_count   # => 0

MR acceptance checklist

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