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>

package.json

@@ -21,6 +21,12 @@
     "typecheck": "tsc -b",
     "test": "pnpm --filter \"./packages/*\" --filter \"./apps/*\" -r --if-present test",
     "test:e2e": "pnpm --filter @ketopath/web test:e2e",
+    "db:generate": "pnpm --filter @ketopath/db db:generate",
+    "db:migrate": "pnpm --filter @ketopath/db db:migrate",
+    "db:migrate:deploy": "pnpm --filter @ketopath/db db:migrate:deploy",
+    "db:studio": "pnpm --filter @ketopath/db db:studio",
+    "db:format": "pnpm --filter @ketopath/db db:format",
+    "db:seed": "pnpm --filter @ketopath/db db:seed",
     "prepare": "husky"
   },
   "devDependencies": {