feat(db): encrypt Profile health fields at rest (ADR 0002)

Wire prisma-field-encryption AES-256-GCM extension on the shared Prisma
client and annotate the four sensitive columns on Profile with @encrypted:
- weightStartKg / weightCurrentKg / weightGoalKg (Decimal → String)
- targetDate (DateTime @db.Date → String, ISO YYYY-MM-DD)

Other Profile fields stay in clear text per ADR 0002 (age, gender,
heightCm, activityLevel) — they're needed for plan generation and
aggregate analytics, and are not strongly identifying on their own.

apps/api profile.routes.ts:
- serialize() now reads the columns as strings and parses them back to
  numbers for BMR/TDEE; targetDate is already an ISO string from the DB
- the upsert stringifies numeric inputs and slices the date to YYYY-MM-DD

Env wiring:
- packages/db, apps/api, apps/web .env.example all document
  PRISMA_FIELD_ENCRYPTION_KEY (k1.aesgcm256.<base64url>) — must match
  across every process that hits the DB
- key generation snippet documented inline

Migration is intentionally NOT in this commit: needs to be created against
a live Postgres instance and applied. The fields change Decimal/Date → text
so prisma migrate dev will require a USING cast — see the follow-up commit.

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

apps/api/.env.example

@@ -8,6 +8,11 @@ CORS_ORIGINS=http://localhost:3000
 
 DATABASE_URL=postgresql://luciano:luciano%40@localhost:5432/ketopath?schema=public
 
+# Cifratura at-rest dei campi sanitari (vedi ADR 0002).
+# Genera con: node -e "console.log('k1.aesgcm256.'+require('crypto').generateKeySync('aes',{length:256}).export().toString('base64url'))"
+# DEVE coincidere con apps/web/.env e con tutti i deploy che leggono il DB.
+PRISMA_FIELD_ENCRYPTION_KEY=
+
 # Better Auth — DEVE coincidere con apps/web/.env (sessione condivisa)
 BETTER_AUTH_SECRET=
 BETTER_AUTH_URL=http://localhost:3000