Add scholarship decline flow with admin email and totals exclusion

[maebeale]

🤖 suggested review level: 5 Inspect 🔬 new decline state that alters financial totals across dashboards, grant budgets, revenue/scholarship reports, and many display surfaces

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

• Recipients can now say No to a scholarship agreement (with a reason) from their scholarship page, not just Agree.
• Declining emails the admin team an FYI (via the existing Notification pipeline), records the reason, and removes the award from every total so declined money no longer inflates dashboards, grant budgets, or reports.
• The scholarship page also now shows how much the registrant will owe after the scholarship is applied (event cost − all allocations).

How did you approach the change?

• Decline flow: new decline_agreement action + route; native <details> disclosure reveals a required reason box next to Agree. Scholarship#decline_agreement! stamps agreement_declined_at/reason, clears any signed state, and zeroes the allocation so all allocation-based totals self-correct.
• Admin email: new scholarship_agreement_declined_fyi Notification kind → NotificationMailer method + HTML/text views + preview, wired through NotificationMailerJob.
• Totals exclusion (two levers): zeroed allocation covers raw-allocation totals; .not_declined added to the amount_cents/scope readers — grant budget/remaining, funding split scopes, EventDashboard, EventScholarshipFigures, EventRevenueFigures, ScholarshipsGrouping, grant decorator counts, registrant award badge.
• Declined shown everywhere: scholarships index rows, event registration card, grant scholarships table, event recipient card, onboarding row, admin scholarship show/edit. Reusable badge helpers on ScholarshipDecorator.
• Re-offer + guard: editing the award amount clears the decline (and re-funds the allocation); a new scholarship-decline-guard Stimulus controller warns the admin before saving that change so the decline isn't discarded silently.
• History: no new model — the decline is captured by the denormalized status, the FYI Notification (shown in the scholarship's Communications box), and Ahoy events (linked from the edit page's audit info).

Anything else to add?

• Migration adds agreement_declined_at + agreement_declined_reason to scholarships.
• Open question for review: reinstating a decline currently happens only via an amount edit. Toggling "Agreement signed" without changing the amount leaves the allocation at $0. See PR discussion.

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