Drop workshops.sign_up_url #3018

Problem

workshops.sign_up_url is a nullable string. Its only purpose was an external RSVP link shown in workshop partials; the last read was removed in c9391a35 (2014-09-09) when the workshop page moved to invitation-based RSVP. Nothing has read or written it since. Production snapshot (2026-10-12): 3 non-null values of 2603 rows, all set before 2014.

Proposed change

Two-step, as with #3007:

1. PR 1: add sign_up_url to self.ignored_columns on Workshop, deploy, soak.
2. PR 2: safety_assured { remove_column :workshops, :sign_up_url, :string }, plus regenerate schema.rb.

Acceptance

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

Labels: refactoring, trivial