Initialize conversion of epic_user_mentions.note_id to bigint

What does this MR do and why?

Initialize the conversion of epic_user_mentions.note_id from integer to bigint, as it references notes.id.

This MR creates the new bigint column, installs the trigger to keep it in sync for new records, and queues batched background migration to back-fill the existing records.

#389306 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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