feat(#353): Create VaultService NestJS module for OpenBao Transit

Implements secure credential encryption using OpenBao Transit API with
automatic fallback to AES-256-GCM when OpenBao is unavailable.

Features:
- AppRole authentication with automatic token renewal at 50% TTL
- Transit encrypt/decrypt with 4 named keys
- Automatic fallback to CryptoService when OpenBao unavailable
- Auto-detection of ciphertext format (vault:v1: vs AES)
- Request timeout protection (5s default)
- Health indicator for monitoring
- Backward compatible with existing AES-encrypted data

Security:
- ERROR-level logging for fallback
- Proper error propagation (no silent failures)
- Request timeouts prevent hung operations
- Secure credential file reading

Migrations:
- Account encryption middleware uses VaultService
- Uses TransitKey.ACCOUNT_TOKENS for OAuth tokens
- Backward compatible with existing encrypted data

Tests: 56 tests passing (36 VaultService + 20 middleware)

Closes #353

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

apps/api/src/vault/vault.module.ts

@@ -0,0 +1,19 @@
+/**
+ * Vault Module
+ *
+ * Global module providing OpenBao Transit encryption services.
+ */
+
+import { Module, Global } from "@nestjs/common";
+import { ConfigModule } from "@nestjs/config";
+import { VaultService } from "./vault.service";
+import { VaultHealthIndicator } from "./vault.health";
+import { CryptoService } from "../federation/crypto.service";
+
+@Global()
+@Module({
+  imports: [ConfigModule],
+  providers: [VaultService, VaultHealthIndicator, CryptoService],
+  exports: [VaultService, VaultHealthIndicator],
+})
+export class VaultModule {}