fix(portal): portal_admin.py runnable as a script, not just -m

`python3 backend/portal_admin.py` set sys.path[0] to backend/, hiding the
`backend` package and breaking `from backend.database import ...`. Insert the
project root on sys.path so the documented script invocation works.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/portal_admin.py

@@ -31,6 +31,10 @@ import secrets
 import argparse
 from datetime import datetime
 
+# Allow `python3 backend/portal_admin.py ...` (which puts backend/ on sys.path[0],
+# hiding the `backend` package) in addition to `python3 -m backend.portal_admin`.
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
 from backend.database import SessionLocal
 from backend.models import Client, ClientAccessToken, Project
 from backend.portal_auth import hash_token