fix(ocr): log warning when user ID resolution fails

The resolveUserId() catch block was silently swallowing exceptions,
making auth failures invisible in logs.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/controller/OcrController.java

@@ -81,6 +81,7 @@ public class OcrController {
             AppUser user = userService.findByUsername(authentication.getName());
             return user != null ? user.getId() : null;
         } catch (Exception e) {
+            log.warn("Failed to resolve user ID for authentication: {}", authentication.getName(), e);
             return null;
         }
     }