fix: suppress ERROR logging for missing .env files in image update scan (#936)

* fix: suppress ERROR logging for missing .env files in image update scan

The ImageUpdateService logged a full ERROR stack trace for every stack
without a .env file, which is a normal and expected configuration.
Also added a 5-minute check timeout, developer_mode diagnostic logging,
and proper startup timeout cleanup.

* fix: add missing Node fields in test mock to satisfy tsc strict checking

* fix: remove unused variables to satisfy ESLint no-unused-vars

backend/src/services/FileSystemService.ts

@@ -185,8 +185,10 @@ export class FileSystemService {
     try {
       return await fsPromises.readFile(envPath, 'utf-8');
     } catch (error) {
-      console.error('Error reading env file:', error);
-      throw new Error(`Failed to read env file for stack: ${stackName}`);
+      if ((error as NodeJS.ErrnoException).code !== 'ENOENT') {
+        console.error('Error reading env file:', error);
+      }
+      throw error;
     }
   }