Add Pay for Others submitters to the event bulk-reminder page

[maebeale]

🤖 suggested review level: 3 Read 📖 contained logic — new service + mailer + a section on the reminder page, no migrations or data transforms

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

• Admins chase unpaid balances from the event Send bulk emails page, but the payer behind a Pay for Others submission was invisible there — nobody nudged them when attendees they said they'd cover never registered or were never paid for.
• Surfaces those submitters in their own section so the admin can remind the payer in the same send.

How did you approach the change?

• BulkPaymentReminderRecipients service picks the qualifying submitters: any attendee not connected to a registration, or a matched registration not paid in full. Reuses FormSubmissionDecorator#matched_attendees so "connected" means the same thing as the bulk-payments dashboard.
• New "Pay for Others submitters" section at the bottom of the recipient list — separate table, own select-all, per-row not registered / unpaid chips, and a link to the payer's card on the bulk-payments dashboard (new tab).
• Payers get a payer-appropriate email (EventMailer#event_bulk_payment_reminder) linking to their payment ticket, not the registrant "view your ticket" reminder. Shares the admin's custom subject/message; new event_bulk_payment_reminder notification kind so it lands in the payer's comms history.
• Confirm interstitial lists both groups and previews both emails; the batch FYI now counts "recipients" and tags submitters.
• Edge case: an event with qualifying submitters but no emailable registrants still shows the page, and the live preview falls back to a submitter sample.

Anything else to add?

• Selection default: submitters are their own group with their own select-all and are pre-checked by default; registrant filters don't remove or re-check them (avoids stale state under the turbo-frame filter refresh). The confirm step lists exactly who will be emailed before sending.
• Tests: service spec, mailer spec, and request specs covering listing/omitting, confirm, and sending (with, without, and only submitters).

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