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>

packages/db/package.json

@@ -7,6 +7,7 @@
   "scripts": {
     "generate": "drizzle-kit generate",
     "migrate": "drizzle-kit migrate",
+    "seed": "tsx src/seed.ts",
     "studio": "drizzle-kit studio",
     "typecheck": "tsc --noEmit"
   },
@@ -17,6 +18,7 @@
   "devDependencies": {
     "@types/node": "^22.10.7",
     "drizzle-kit": "^0.30.4",
+    "tsx": "^4.19.0",
     "typescript": "^5.7.3"
   }
 }