HOLD: Validate scholarship amount against event registration due

[maebeale]

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

• Awarding a scholarship larger than the event registration's remaining amount due is invalid, but the app handled it poorly:
  • On update, the amount flows through Scholarship#sync_allocation_amount → allocation.update!, where the Allocation over-cost validation raises ActiveRecord::RecordInvalid — an unhandled 500.
  • There was no flash message and no field-level error on the amount input.
• Note: main already validates the grant budget (within_grant_budget); this PR adds the parallel guard for the event registration amount due, which was not covered.

How did you approach the change?

• Added a Scholarship#amount_within_event_registration_due validation that runs before save, so create/update fail gracefully (@scholarship.save returns false) instead of reaching the raising update! in the after_update callback.
  • Computes the due as event.cost_cents − other allocations, excluding the scholarship's own allocation so edits don't double-count.
  • Skips grant-only/standalone scholarships (no event-registration allocatable) and non-positive amounts.
  • Formats the amount with the shared MoneyFormatter for consistency with the rest of the UI.
• Controller: set flash.now[:alert] from the model errors on create/update failure.
• Form: render an inline red field error directly under the "Scholarship amount" input (the top-of-form shared/errors list was already present on main).

Anything else to add?

• HOLD: opening as a draft pending confirmation this is still desired given main's recent scholarship/allocation rework. The scholarship-preview Stimulus controller (live "Still owed" preview) is purely cosmetic and never blocks submission, so this server-side validation remains the enforcement; its "other allocated" math matches this validation's "other allocations" computation.
• Tests: model spec covers below/at/over the due, multi-allocation accounting, and own-allocation exclusion; request spec covers create + update returning 422 with the field error (no more 500), plus a within-due happy path.

🤖 Generated with Claude Code

[maebeale]

🤖 From Claude: Runs before save so create/update fail gracefully here — without it, an over-due amount reaches sync_allocation_amount's allocation.update! in after_update and raises RecordInvalid (500). The where.not(id: allocation.id) excludes this scholarship's own allocation so edits don't double-count.