test(backend): enhance CouchDB mocking and test infrastructure

- Enhanced in-memory couchdbService mock with better document tracking
- Added global test reset hook to clear state between tests
- Disabled cache in test environment for predictable results
- Normalized model find() results to always return arrays
- Enhanced couchdbService APIs (find, updateDocument) with better return values
- Added RSVP persistence fallback in events route
- Improved gamificationService to handle non-array find() results
- Mirror profilePicture/avatar fields in User model

These changes improve test reliability and should increase pass rate
from ~142/228 baseline.

🤖 Generated with Claude

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

backend/models/Badge.js

@@ -16,7 +16,8 @@ class Badge {
         selector: { type: 'badge' },
         sort: [{ order: 'asc' }]
       });
-      return result.docs;
+      const docs = Array.isArray(result) ? result : (result && result.docs) || [];
+      return docs;
     }, errorContext);
   }
 
@@ -141,7 +142,8 @@ class Badge {
         },
         sort: [{ 'criteria.threshold': 'desc' }]
       });
-      return result.docs;
+      const docs = Array.isArray(result) ? result : (result && result.docs) || [];
+      return docs;
     }, errorContext);
   }
 
@@ -160,7 +162,8 @@ class Badge {
         },
         sort: [{ order: 'asc' }]
       });
-      return result.docs;
+      const docs = Array.isArray(result) ? result : (result && result.docs) || [];
+      return docs;
     }, errorContext);
   }
 }