feat: tip and payment splitting between staff roles

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>

packages/db/migrations/meta/_journal.json

@@ -50,6 +50,13 @@
       "when": 1773783600000,
       "tag": "0006_pet_profile_attributes",
       "breakpoints": true
+    },
+    {
+      "idx": 7,
+      "version": "7",
+      "when": 1773820800000,
+      "tag": "0007_tip_splitting",
+      "breakpoints": true
     }
   ]
 }