Add notes field to online scheduler

Add a notes field to the online (public) scheduler so the person booking can leave instructions for the office.
When notes are submitted, they should appear in the activity feed as a pinned item and tag @office so scheduling teams see them immediately.
The same text should be available as an email template variable for office notification emails when someone books online.

Implementation reviews described this as a cross-company need, not a one-off configuration.
Internal scheduler already has sticky notes patterns (src/components/scheduling/NewSchedulingForm.tsx, SchedulerNotesPanel)—online booking is a separate UX under src/app/scheduler/ (e.g. SlotsStep.tsx, ServiceConfirmation.tsx).
Persisted booking likely creates or updates quotes/inspections via backend routes—activity pinning should follow existing patterns for inspection activity (recordPendingContext, activity feed subscribers) where applicable.
In the Monday kickoff, the team also called out a downstream requirement: users will likely want these scheduler notes to be available as a merge variable in office-facing emails or action flows, not only shown in the activity feed. The implementation may need to balance note/activity-feed storage with structured reuse in notifications.

Frontend (attik-frontend)

src/app/scheduler/ — add field to the appropriate step(s), validation, and pass-through to the booking API payload.
Wire template variable documentation in email-builder settings if variables are registered centrally.

Backend (attik-backend)

Booking/create-inspection handlers (quote or inspection creation from online scheduler)—accept notes, store on the right document, emit activity with pin + mention metadata.
Email variable registry / merge pipeline for notifications.

attik-frontend/src/app/scheduler/slots/SlotsStep.tsx
attik-frontend/src/app/scheduler/services/ServiceConfirmation.tsx
attik-backend/src/util/functions/inspection/createInspection.ts (related creation paths as appropriate)