Debug: Ajouter logs détaillés pour authentification

Problème: Login échoue avec 401, besoin de debug
Ajout logs INFO pour:
- Email fourni vs attendu
- Comparaison email
- Longueur des mots de passe
- Résultat authentification

À retirer en production une fois le problème résolu.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

backend/src/core/auth.py

@@ -100,13 +100,22 @@ def authenticate_user(email: str, password: str) -> Optional[dict]:
     Returns:
         User data if authenticated, None otherwise
     """
+    # Debug logging (remove in production)
+    logger.info(f"Auth attempt - Email provided: '{email}'")
+    logger.info(f"Auth attempt - Expected email: '{settings.ADMIN_EMAIL}'")
+    logger.info(f"Auth attempt - Email match: {email == settings.ADMIN_EMAIL}")
+    logger.info(f"Auth attempt - Password length: {len(password)}")
+    logger.info(f"Auth attempt - Expected password length: {len(settings.ADMIN_PASSWORD)}")
+
     # Check against admin credentials from environment
     if email == settings.ADMIN_EMAIL and password == settings.ADMIN_PASSWORD:
+        logger.info(f"✅ Authentication successful for {email}")
         return {
             "email": email,
             "role": "admin"
         }
 
+    logger.warning(f"❌ Authentication failed for {email}")
     return None