refactor(auth): Use user_id as JWT sub and implement get_user_by_id

Switched from username to user_id as the primary identifier in JWT tokens to better support external authentication providers. Added get_user_by_id to HistoryManager and updated API dependencies and tests to reflect these changes.

src/ea_chatbot/history/manager.py

@@ -39,6 +39,11 @@ class HistoryManager:
             result = session.execute(select(User).where(User.username == email))
             return result.scalar_one_or_none()
 
+    def get_user_by_id(self, user_id: str) -> Optional[User]:
+        """Fetch a user by their ID."""
+        with self.get_session() as session:
+            return session.get(User, user_id)
+
     def create_user(self, email: str, password: Optional[str] = None, display_name: Optional[str] = None) -> User:
         """Create a new local user."""
         hashed_password = ph.hash(password) if password else None