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/User.js

@@ -25,6 +25,7 @@ class User {
 
     // --- Profile Information ---
     this.avatar = data.avatar || null;
+    this.profilePicture = data.profilePicture || data.avatar || null;
     this.cloudinaryPublicId = data.cloudinaryPublicId || null;
     this.bio = data.bio || "";
     if (this.bio.length > 500) { throw new ValidationError("Bio cannot exceed 500 characters.", "bio", this.bio); }
@@ -198,6 +199,7 @@ class User {
       email: this.email,
       password: this.password,
       avatar: this.avatar,
+      profilePicture: this.profilePicture,
       cloudinaryPublicId: this.cloudinaryPublicId,
       bio: this.bio,
       location: this.location,