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>

package.json

@@ -9,7 +9,8 @@
     "lint": "pnpm -r lint",
     "typecheck": "pnpm -r typecheck",
     "test": "pnpm -r test",
-    "db:migrate": "pnpm --filter @groombook/db migrate"
+    "db:migrate": "pnpm --filter @groombook/db migrate",
+    "db:seed": "pnpm --filter @groombook/db seed"
   },
   "engines": {
     "node": ">=20",