backend: complete P0 session rotation hardening

TODO.md

@@ -4,10 +4,10 @@ Concrete follow-up work for Jellomator, prioritized by implementation risk and u
 
 ## P0 - Security and Reliability
 
-- [ ] Add session expiry and rotation.
+- [x] Add session expiry and rotation.
   - [x] Add `expires_at` and `last_seen_at` to `sessions`.
   - [x] Reject expired tokens in `current_user`.
-  - [ ] Rotate session token on login and periodically on use.
+  - [x] Rotate session token on login and periodically on use.
 - [x] Harden auth endpoints.
   - [x] Add login rate limiting by IP + username pair.
   - [x] Add brute-force lockout window with clear error message.

backend/main.py

@@ -194,22 +194,21 @@ def current_user(request: Request, response: Response | None = None):
                     should_rotate = (now - datetime.fromisoformat(last_seen_at)).total_seconds() >= SESSION_ROTATE_SECONDS
                 except ValueError:
                     should_rotate = True
-            if should_rotate:
+            if should_rotate and response is not None:
                 new_token = secrets.token_urlsafe(32)
                 cur.execute(
                     "update sessions set token=%s,last_seen_at=%s,expires_at=%s where token=%s",
                     (new_token, now_iso, new_expires_at, token),
                 )
-                if response is not None:
-                    response.set_cookie(
-                        SESSION_COOKIE,
-                        new_token,
-                        httponly=True,
-                        samesite="lax",
-                        secure=SESSION_COOKIE_SECURE,
-                        max_age=SESSION_TTL_SECONDS,
-                        path="/",
-                    )
+                response.set_cookie(
+                    SESSION_COOKIE,
+                    new_token,
+                    httponly=True,
+                    samesite="lax",
+                    secure=SESSION_COOKIE_SECURE,
+                    max_age=SESSION_TTL_SECONDS,
+                    path="/",
+                )
             else:
                 cur.execute(
                     "update sessions set last_seen_at=%s,expires_at=%s where token=%s",
@@ -358,6 +357,10 @@ def login(request: Request, inp: LoginIn):
             raise HTTPException(401, "Invalid credentials")
         login_attempts.pop(key, None)
         login_lockouts.pop(key, None)
+        old_token = request.cookies.get(SESSION_COOKIE)
+        if old_token:
+            with c.cursor() as cur:
+                cur.execute("delete from sessions where token=%s", (old_token,))
         token = secrets.token_urlsafe(32)
         with c.cursor() as cur:
             now = utc_now_iso()