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/package.json

@@ -10,10 +10,25 @@
   },
   "scripts": {
     "typecheck": "tsc --noEmit",
-    "lint": "eslint src --max-warnings=0"
+    "lint": "eslint src --max-warnings=0",
+    "db:generate": "prisma generate",
+    "db:migrate": "prisma migrate dev",
+    "db:migrate:deploy": "prisma migrate deploy",
+    "db:studio": "prisma studio",
+    "db:format": "prisma format",
+    "db:seed": "tsx prisma/seed.ts"
+  },
+  "dependencies": {
+    "@prisma/client": "^5.18.0"
   },
   "devDependencies": {
     "@ketopath/eslint-config": "workspace:*",
-    "@ketopath/tsconfig": "workspace:*"
+    "@ketopath/tsconfig": "workspace:*",
+    "prisma": "^5.18.0",
+    "tsx": "^4.16.2"
+  },
+  "prisma": {
+    "schema": "prisma/schema.prisma",
+    "seed": "tsx prisma/seed.ts"
   }
 }