feat(server): add global encryption module (AES-256-GCM)

Provides an injectable EncryptionService that encrypts/decrypts strings
with AES-256-GCM using a key derived from APP_SECRET with domain
separation. Payloads are base64 JSON envelopes with validated IV and
auth tag lengths, and failures surface as typed errors.

apps/server/src/integrations/encryption/encryption.module.ts

@@ -0,0 +1,9 @@
+import { Global, Module } from '@nestjs/common';
+import { EncryptionService } from './encryption.service';
+
+@Global()
+@Module({
+  providers: [EncryptionService],
+  exports: [EncryptionService],
+})
+export class EncryptionModule {}