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>

packages/db/package.json

@@ -9,6 +9,10 @@
     ".": {
       "default": "./dist/index.js",
       "types": "./src/index.ts"
+    },
+    "./factories": {
+      "default": "./src/factories.ts",
+      "types": "./src/factories.ts"
     }
   },
   "scripts": {
@@ -16,6 +20,7 @@
     "generate": "drizzle-kit generate",
     "migrate": "drizzle-kit migrate",
     "seed": "tsx src/seed.ts",
+    "reset": "tsx src/reset.ts && drizzle-kit migrate && tsx src/seed.ts",
     "studio": "drizzle-kit studio",
     "typecheck": "tsc --noEmit"
   },