feat(db): scaffold Prisma schema with User, Profile, Preferences

Initial schema covers only the 3 base entities from PRD §8 — additional
models (MealPlan, Recipe, WeightEntry, FastEvent, ShoppingList, etc.)
will be added alongside their respective features.

- Postgres datasource via DATABASE_URL
- Enums: Role, Gender, ActivityLevel, Phase, CookingTime, FastingProtocol
- Singleton PrismaClient export from @ketopath/db
- Root scripts: db:generate / db:migrate / db:studio / db:seed / db:format
- Seed stub at prisma/seed.ts
- Ignore .claude/settings.local.json (per-user CLI permissions)

The first migration must be run by the developer:
  pnpm db:migrate --name init

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/db/src/index.ts

@@ -1 +1,11 @@
-export {};
+export { prisma } from './client.js';
+export {
+  ActivityLevel,
+  CookingTime,
+  FastingProtocol,
+  Gender,
+  Phase,
+  Prisma,
+  Role,
+} from '@prisma/client';
+export type { Preferences, Profile, User } from '@prisma/client';