feat(#352): Encrypt existing plaintext Account tokens

Implements transparent encryption/decryption of OAuth tokens via Prisma middleware with progressive migration strategy.

Core Implementation:
- Prisma middleware transparently encrypts tokens on write, decrypts on read
- Auto-detects ciphertext format: aes:iv:authTag:encrypted, vault:v1:..., or plaintext
- Uses existing CryptoService (AES-256-GCM) for encryption
- Progressive encryption: tokens encrypted as they're accessed/refreshed
- Zero-downtime migration (schema change only, no bulk data migration)

Security Features:
- Startup key validation prevents silent data loss if ENCRYPTION_KEY changes
- Secure error logging (no stack traces that could leak sensitive data)
- Graceful handling of corrupted encrypted data
- Idempotent encryption prevents double-encryption
- Future-proofed for OpenBao Transit encryption (Phase 2)

Token Fields Encrypted:
- accessToken (OAuth access tokens)
- refreshToken (OAuth refresh tokens)
- idToken (OpenID Connect ID tokens)

Backward Compatibility:
- Existing plaintext tokens readable (encryptionVersion = NULL)
- Progressive encryption on next write
- BetterAuth integration transparent (middleware layer)

Test Coverage:
- 20 comprehensive unit tests (89.06% coverage)
- Encryption/decryption scenarios
- Null/undefined handling
- Corrupted data handling
- Legacy plaintext compatibility
- Future vault format support
- All CRUD operations (create, update, updateMany, upsert)

Files Created:
- apps/api/src/prisma/account-encryption.middleware.ts
- apps/api/src/prisma/account-encryption.middleware.spec.ts
- apps/api/prisma/migrations/20260207_encrypt_account_tokens/migration.sql

Files Modified:
- apps/api/src/prisma/prisma.service.ts (register middleware)
- apps/api/src/prisma/prisma.module.ts (add CryptoService)
- apps/api/src/federation/crypto.service.ts (add key validation)
- apps/api/prisma/schema.prisma (add encryptionVersion)
- .env.example (document ENCRYPTION_KEY)

Fixes #352

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -90,6 +90,17 @@ AUTHENTIK_PORT_HTTPS=9443
 JWT_SECRET=REPLACE_WITH_RANDOM_SECRET_MINIMUM_32_CHARS
 JWT_EXPIRATION=24h
 
+# ======================
+# Encryption (Credential Security)
+# ======================
+# CRITICAL: Generate a random 32-byte (256-bit) encryption key
+# This key is used for AES-256-GCM encryption of OAuth tokens and sensitive data
+# Command to generate: openssl rand -hex 32
+# SECURITY: Never commit this key to version control
+# SECURITY: Use different keys for development, staging, and production
+# SECURITY: Store production keys in a secure secrets manager (see docs/design/credential-security.md)
+ENCRYPTION_KEY=REPLACE_WITH_64_CHAR_HEX_STRING_GENERATE_WITH_OPENSSL_RAND_HEX_32
+
 # ======================
 # Ollama (Optional AI Service)
 # ======================