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/prisma/seed.ts

@@ -0,0 +1,20 @@
+import { PrismaClient } from '@prisma/client';
+
+const prisma = new PrismaClient();
+
+async function main(): Promise<void> {
+  // TODO: dati di seed (utenti demo, ricette base, ecc.) — verranno aggiunti
+  // quando le entità relative saranno modellate.
+  // eslint-disable-next-line no-console
+  console.info('[seed] nothing to seed yet');
+}
+
+main()
+  .catch((err: unknown) => {
+    // eslint-disable-next-line no-console
+    console.error(err);
+    process.exitCode = 1;
+  })
+  .finally(() => {
+    void prisma.$disconnect();
+  });