fix(#289): Prevent private key decryption error data leaks

Modified decrypt() error handling to only log error type without
stack traces, error details, or encrypted content. Added test to
verify sensitive data is not exposed in logs.

Security improvement: Prevents leakage of encrypted data or partial
decryption results through error logs.

Fixes #289

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/api/src/federation/crypto.service.ts

@@ -90,7 +90,10 @@ export class CryptoService {
 
       return decrypted;
     } catch (error) {
-      this.logger.error("Decryption failed", error);
+      // Security: Do not log error details which may contain sensitive data
+      // Only log error type/code without stack trace or encrypted content
+      const errorType = error instanceof Error ? error.constructor.name : "Unknown";
+      this.logger.error(`Decryption failed: ${errorType}`);
       throw new Error("Failed to decrypt data");
     }
   }