Security: Enhance encryption configuration robustness

[MrSebastian]

          _:warning: Potential issue_

Security: Enhance encryption configuration robustness

The current configuration has several security considerations that should be addressed:

1. Explicitly specify the cipher transformation (mode and padding)
2. Consider using a stronger key size (AES-256)
3. Implement proper key derivation

Consider updating the configuration as follows:

-    @Value("${service.config.crypto.key}")
-    String key;
+    @Value("${service.config.crypto.key}")
+    private String rawKey;
+    
+    private static final String CIPHER_TRANSFORMATION = "AES/GCM/NoPadding";
+    private static final int KEY_SIZE_BYTES = 32; // For AES-256
+    
+    private byte[] deriveKey() {
+        // Use a proper key derivation function (e.g., PBKDF2)
+        return Arrays.copyOf(rawKey.getBytes(), KEY_SIZE_BYTES);
+    }

Committable suggestion skipped: line range outside the PR's diff.

Originally posted by @coderabbitai[bot] in #635 (comment)