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>

packages/db/src/client.ts

@@ -1,13 +1,19 @@
 import { PrismaClient } from '@prisma/client';
+import { fieldEncryptionExtension } from 'prisma-field-encryption';
 
+// Singleton-friendly storage of the BASE client (so HMR doesn't open a new pool
+// every time). The exported `prisma` is the EXTENDED client returned by
+// $extends — it wraps the base client with the field-encryption middleware.
 const globalForPrisma = globalThis as unknown as { prisma?: PrismaClient };
 
-export const prisma: PrismaClient =
+const baseClient =
   globalForPrisma.prisma ??
   new PrismaClient({
     log: process.env.NODE_ENV === 'development' ? ['warn', 'error'] : ['error'],
   });
 
 if (process.env.NODE_ENV !== 'production') {
-  globalForPrisma.prisma = prisma;
+  globalForPrisma.prisma = baseClient;
 }
+
+export const prisma = baseClient.$extends(fieldEncryptionExtension());