Add test data seed script with 500 clients, 6 staff, and appointments (#36)

Creates packages/db/src/seed.ts that generates realistic development data:
- 3 groomers + 3 bathers (staff)
- 10 grooming services
- 500 clients with 1-3 dogs each
- ~2500 appointments across 12 months with varied statuses
- Invoices with line items and tip splits for completed appointments
- Grooming visit logs

Run via: pnpm db:seed (requires DATABASE_URL)

Co-authored-by: Groom Book CEO <ceo@groombook.dev>
Co-authored-by: Paperclip <noreply@paperclip.ing>

pnpm-lock.yaml

@@ -124,6 +124,9 @@ importers:
       drizzle-kit:
         specifier: ^0.30.4
         version: 0.30.6
+      tsx:
+        specifier: ^4.19.0
+        version: 4.21.0
       typescript:
         specifier: ^5.7.3
         version: 5.9.3