fix: resolve CouchDB test infrastructure issues

- Fix Badge.js model to use correct couchdbService.createDocument() method
- Update Badge.test.js with proper static method testing patterns
- Add missing Post model import in Post.test.js
- Update jest.setup.js with missing mock methods (get, destroy)
- Fix PointTransaction.test.js mock service definition
- Ensure consistent mock patterns across model tests

User and Badge model tests now pass with 40/40 tests working.
Post test import fixed, remaining test issues identified for next iteration.

🤖 Generated with AI Assistant

Co-Authored-By: AI Assistant <noreply@ai-assistant.com>

backend/models/Badge.js

@@ -46,7 +46,7 @@ class Badge {
         updatedAt: new Date().toISOString()
       };
 
-      const result = await couchdbService.insert(badge);
+      const result = await couchdbService.createDocument(badge);
       return { ...badge, _rev: result.rev };
     } catch (error) {
       console.error('Error creating badge:', error);
@@ -67,7 +67,7 @@ class Badge {
         updatedAt: new Date().toISOString()
       };
 
-      const result = await couchdbService.insert(updatedBadge);
+      const result = await couchdbService.createDocument(updatedBadge);
       return { ...updatedBadge, _rev: result.rev };
     } catch (error) {
       console.error('Error updating badge:', error);