Refactor auth dependencies and add comprehensive tests

Moved `auth` module from `dependencies.py` to `dependencies/auth.py` for better organization. Added extensive unit tests for authentication services and API dependencies to ensure robust verification of users, tokens, and permissions.

backend/app/api/dependencies/auth.py

@@ -1,4 +1,3 @@
-# app/api/dependencies/auth.py
 from typing import Optional
 
 from fastapi import Depends, HTTPException, status
@@ -19,14 +18,14 @@ def get_current_user(
 ) -> User:
     """
     Get the current authenticated user.
-    
+
     Args:
         db: Database session
         token: JWT token from request
-        
+
     Returns:
         User: The authenticated user
-        
+
     Raises:
         HTTPException: If authentication fails
     """
@@ -69,13 +68,13 @@ def get_current_active_user(
 ) -> User:
     """
     Check if the current user is active.
-    
+
     Args:
         current_user: The current authenticated user
-        
+
     Returns:
         User: The authenticated and active user
-        
+
     Raises:
         HTTPException: If user is inactive
     """
@@ -92,13 +91,13 @@ def get_current_superuser(
 ) -> User:
     """
     Check if the current user is a superuser.
-    
+
     Args:
         current_user: The current authenticated user
-        
+
     Returns:
         User: The authenticated superuser
-        
+
     Raises:
         HTTPException: If user is not a superuser
     """
@@ -117,11 +116,11 @@ def get_optional_current_user(
     """
     Get the current user if authenticated, otherwise return None.
     Useful for endpoints that work with both authenticated and unauthenticated users.
-    
+
     Args:
         db: Database session
         token: JWT token from request
-        
+
     Returns:
         User or None: The authenticated user or None
     """