Add import_jobs_concurrency_limit to application settings

What does this MR do and why?

This setting is currently a no-op and will be wired up in follow-up MRs

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Start a local GDK instance.

2. Open a Rails console: gdk rails console

3. Confirm the default value is applied:

   ApplicationSetting.current.import_jobs_concurrency_limit
   # => 100

4. Update the setting and verify it persists:

   ApplicationSetting.current.update!(import_jobs_concurrency_limit: 50)
   ApplicationSetting.current.reload.import_jobs_concurrency_limit
   # => 50

5. Confirm the setting is exposed via the API:

   curl --header "PRIVATE-TOKEN: <your_token>" \
     "http://gdk.test:3000/api/v4/application/settings" | \
     jq '.import_jobs_concurrency_limit'
   # => 50

6. Update it via the API and verify the change:

   curl --request PUT \
     --header "PRIVATE-TOKEN: <your_token>" \
     --data "import_jobs_concurrency_limit=75" \
     "http://gdk.test:3000/api/v4/application/settings" | \
     jq '.import_jobs_concurrency_limit'
   # => 75

7. Delete the setting otherwise your application setting will become invalid if you change branches

a = ApplicationSetting.first
a.importers.delete("import_jobs_concurrency_limit")
a.save!

MR acceptance checklist

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