chore(workshops): drop the dead sign_up_url column

[mroderick]

Part of #3018, stacked on the ignore PR (base = its head branch; when that merges, GitHub retargets this PR to master). Closes #3018 once merged after the ignore PR ships and soaks in production.

Removes sign_up_url from workshops with safety_assured { remove_column }, regenerates db/schema.rb, and drops the transient legacy-column specs.

Detail

Two commits mirroring the merged meetings drop: the migration plus schema regeneration, then the spec removal — each green on its own. The column holds 3 non-null values out of 2603 production rows, all set before 2014, and has had no readers since the invitation-based RSVP switch in c9391a35 (2014-09-09). The migration was regenerated from bin/rails db:migrate rather than edited by hand.

Review notes

• Merge order gate: ignore PR first, deploy, soak, then this one. The stacked base enforces the ordering in GitHub; the soak window is operational.
• Deliberately not done: no data migration or backfill — the three stale values go with the column.