fix(digest): return 400 when EMAIL delivery is requested

IllegalArgumentException from the prefs controller used to become a 500
with no message. Map it to a 400 so the UI can show that email delivery
is not available yet.

Co-authored-by: Venkat SF <venkatesh.sakamuri@stayflexi.com>

Author: cursoragent

backend/src/main/java/com/dbaagent/controller/DigestPreferenceController.java

@@ -281,4 +281,9 @@ public record PreferencePreview(String username, String connectionId, String per
     public record SeedResultResponse(int usersProcessed, int preferencesCreated, List<String> skipped) {}
 
     public record DigestStatusResponse(boolean perUserMode, long enabledPreferences, int distinctUsers) {}
+
+    @ExceptionHandler(IllegalArgumentException.class)
+    public ResponseEntity<Map<String, String>> handleBadRequest(IllegalArgumentException e) {
+        return ResponseEntity.badRequest().body(Map.of("message", e.getMessage() != null ? e.getMessage() : "Bad request"));
+    }
 }