Comments: store user_id as author_id for local_agent comments instead of api_token_id

[HamptonMakes]

Problem

When a comment is created via the API, author_id is set to @api_token.id (the API token's ID), not the user's ID. This forces CommentsHelper#comment_author_name to do a raw SQL join through coplan_api_tokens to resolve the user's name, hardcoding the users table name in the process.

Current behavior

# API controller sets token ID as author:
author_id: @api_token.id

# Helper has to join through token to get user name:
CoPlan.user_class
  .joins("INNER JOIN coplan_api_tokens ON coplan_api_tokens.user_id = users.id")
  .where("coplan_api_tokens.id = ?", comment.author_id)
  .pick(:name)

Proposed fix

Store @api_token.user_id as author_id for local_agent comments. Then the helper can use CoPlan.user_class.find_by(id: comment.author_id) for both human and local_agent types — no join, no hardcoded table name.

The agent_name field (already required for local_agent comments) distinguishes which agent made the comment.

Files involved

• engine/app/controllers/coplan/api/v1/comments_controller.rb — change author_id: @api_token.id → author_id: @api_token.user_id
• engine/app/helpers/coplan/comments_helper.rb — simplify local_agent branch to a direct user lookup
• Backfill migration for any existing comments with token IDs as author_id