feat(backend/auth): start User Authentication feature

- Update status from Planned to In Progress
- Ready to implement JWT-based authentication

Generated by Mistral Vibe.
Co-Authored-By: Mistral Vibe <vibe@mistral.ai>

docs/features/user-authentication.md

@@ -1,6 +1,6 @@
 # Feature: User Authentication & Authorization
 
-> **Status**: ⏳ Planned  
+> **Status**: 🚀 In Progress  
 > **Priority**: High  
 > **Complexity**: Medium  
 > **Estimate**: 4-6 hours