Consolidate CE + art supplies built-ins onto callout rows; drop legacy columns

[maebeale]

🤖 suggested review level: 5 Inspect 🔬 migrations that drop Event columns + removal of two standalone pages + wide callout-system changes

What is the goal of this PR and why is this important?

• Every built-in callout page now renders its intro copy from the materialized callout row's "Callout page text" (CE, handouts, FAQ, scholarship previously accepted the text but silently dropped it).
• Finishes migrating the CE and "art supplies" (formerly event_details) built-ins fully onto their callout rows, so their content is edited in one place like every other built-in.

How did you approach the change?

• Drop the legacy event-level columns ce_hours_details / ce_hours_details_label and event_details / event_details_label (two reversible migrations, no backfill — the ticket system isn't in production use for these and dev data re-seeds).
• Delete the two orphaned standalone pages events#ce_hours and events#details (nothing linked to them; superseded by the callout pages).
• Make art_supplies a content callout so it renders on the generic callout page, and rename its magic_key event_details → art_supplies (default title "Art supplies & what to bring").
• Refactor the scholarship and FAQ pages onto the shared _callout_page layout (removes duplicated chrome).
• Remove the dead editor branch + event_details_card plumbing.

Anything else to add?

• Depends on nothing; split out from Redesign the callout editor cards: Published toggle + Visibility section #1982 (which keeps the editor-card redesign).
• Rebased onto the Forms-removal (Move W-9 to the payment callout and remove the Forms built-in #1975) and CE-color (Color the CE ticket card orange while a CE balance is due #1981) changes.
• No data backfill for renamed magic keys, per product call.

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