feat(security): field encryption AES-256-GCM + random IV (at-rest)

- common/crypto/crypto.util.ts: encrypt/decrypt (GCM, random 96-bit IV), format v1:base64(iv|tag|ct)
- EncryptedTransformer applied to users.name + drivers.vehicle_plate (auto, no service change)
- blindIndex (HMAC) helper for future searchable-field encryption (phone)
- tolerant decrypt for legacy plaintext → no migration needed (varchar holds base64)
- ENCRYPTION_KEY env + boot warning if unset; docs/16-encryption.md
- fixes Siro's documented CBC+fixed-IV flaw

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/.env.example

@@ -24,6 +24,11 @@ QUEUE_PREFIX=tripz_
 
 # ---- Auth ----
 JWT_SECRET=change_me_jwt_secret
+
+# ---- تشفير الحقول الحساسة at-rest (AES-256-GCM) ----
+# 32 بايت بصيغة hex (64 محرف). ولّده: openssl rand -hex 32
+# إلزامي للإنتاج — بدونه يُستخدم مفتاح تطوير غير آمن
+ENCRYPTION_KEY=
 JWT_EXPIRES=15m
 JWT_REFRESH_EXPIRES=30d