M15: synchronize contracts and acceptance

backend/app/core/security.py

@@ -20,6 +20,7 @@ class SessionPayload:
     role: str
     display_name: str
     issued_at: int
+    session_id: str = ""
 
 
 def _sign(data: bytes) -> str:
@@ -27,6 +28,11 @@ def _sign(data: bytes) -> str:
     return base64.urlsafe_b64encode(digest).decode().rstrip("=")
 
 
+def session_token_hash(token: str) -> str:
+    """Return a non-reversible identifier safe to persist for token revocation."""
+    return hashlib.sha256(token.encode()).hexdigest()
+
+
 def create_session_token(payload: SessionPayload) -> str:
     body = json.dumps(payload.__dict__, separators=(",", ":")).encode()
     encoded_body = base64.urlsafe_b64encode(body).decode().rstrip("=")