Add language_id column to programming_languages table

What does this MR do and why?

Contributes to #519895 (closed)

Problem

The programming_languages table uses an auto-increment id that is inconsistent across GitLab Cells. Gitaly returns a globally stable language_id but there is no column to store it yet.

Solution

Add a nullable language_id bigint column to the programming_languages table and a unique index on it. This prepares for later steps that will populate the column from the Gitaly CommitLanguagesResponse and eventually switch the primary key.

References

• Issue: #519895 (closed)
• Gitaly MR that added language_id to the protobuf response: gitaly!8068 (merged)
• Rails MR exposing language_id from Gitaly (step 2): !230678 (merged)

Screenshots or screen recordings

Not applicable — backend-only schema change.

How to set up and validate locally

1. Run the migrations: bundle exec rails db:migrate
2. Verify the column exists:

   ProgrammingLanguage.column_names.include?('language_id') # => true

3. Verify the unique index exists:

   ActiveRecord::Base.connection.indexes(:programming_languages).map(&:name)
   # should include 'index_programming_languages_on_language_id'

4. Verify rollback: bundle exec rails db:rollback STEP=2

MR acceptance checklist

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