9b1ad6c4d8
Implements groombook/groombook#12 — track which staff worked on each pet and calculate tip distribution based on who was involved. Changes: - DB: Add bather_staff_id to appointments (optional secondary staff) - DB: Add invoice_tip_splits table (per-staff tip share ledger) - API: appointments POST/PATCH accept batherStaffId - API: GET /invoices/:id now includes tipSplits[] - API: POST /invoices/:id/tip-splits — saves tip distribution - API: GET /reports/tip-splits — payroll summary of tip earnings - Frontend: Bather/Assistant select on New Appointment form - Frontend: Tip Distribution section in Invoice Detail modal - Auto-populates 70%/30% split when bather is assigned - Editable percentages before payment; saved on Mark as Paid - Displays recorded splits on paid invoices Co-Authored-By: Paperclip <noreply@paperclip.ing>