feat: deterministic seed, impersonation migration, test factories (GRO-110)

Phase 1 — Seed Hardening:
- Replace all Math.random() calls in seed.ts with a Mulberry32 seeded PRNG
  (seed 42) so the same data set is reproduced on every run
- Replace crypto.randomUUID() with a PRNG-based UUID v4 generator
- Add manager (Jordan Lee) and receptionist (Sam Rivera) staff members
  to seed — previously all staff were groomers
- New packages/db/src/reset.ts drops all tables/enums and re-runs
  migrate + seed; exposed as `pnpm db:reset` at root
- Generate migration 0010_impersonation_sessions.sql for the
  impersonation_sessions and impersonation_audit_logs tables that were
  already in schema.ts but had no corresponding migration

Phase 2 — Test Factories:
- New packages/db/src/factories.ts with buildStaff, buildClient, buildPet,
  buildService, buildAppointment and resetFactoryCounters helpers
- Exported via @groombook/db/factories subpath (package.json + vitest alias)
- impersonation.test.ts updated to use buildStaff instead of hand-rolled
  fixture objects

Closes #90 (Phases 1 + 2)

Co-Authored-By: Paperclip <noreply@paperclip.ing>

package.json

@@ -10,7 +10,8 @@
     "typecheck": "pnpm -r typecheck",
     "test": "pnpm -r --filter='!@groombook/e2e' test",
     "db:migrate": "pnpm --filter @groombook/db migrate",
-    "db:seed": "pnpm --filter @groombook/db seed"
+    "db:seed": "pnpm --filter @groombook/db seed",
+    "db:reset": "pnpm --filter @groombook/db reset"
   },
   "engines": {
     "node": ">=20",