Drop events.coach_description #3019

Problem

events.coach_description is a text column from the early event-invites feature. Its reads were removed in de44fe17 (2016-06-14, "Event invites rebased (#461)"), which replaced the coach-specific text in invitation mailers with the general event.description. Nothing reads it today, and it is not on any admin form. Production snapshot (2026-10-12): 3 non-null of 260 rows.

The only write left in the codebase is spec/fabricators/event_fabricator.rb, which sets a Faker sentence; that line goes away with the drop.

Proposed change

Two-step, as with #3007:

1. PR 1: add coach_description to self.ignored_columns on Event and delete the fabricator line.
2. PR 2: safety_assured { remove_column :events, :coach_description, :text }, plus regenerate schema.rb.

Acceptance

• After PR 1: event queries no longer name the column; event fabricator no longer sets it; suite passes.
• After PR 2: column gone from schema.rb; full suite passes.

Labels: refactoring, trivial