chore: bump python-multipart to 0.0.31 for CVE fixes

pip-audit (dependency-audit CI step) flagged python-multipart 0.0.27
for CVE-2026-53538/53539/53540 (fixed in 0.0.30/0.0.31). Bump to
0.0.31. pip-audit clean and backend test suite green.

ha-relevant: no

backend/requirements.txt

@@ -8,7 +8,7 @@ pydantic==2.9.2
 pydantic-settings==2.5.2
 python-jose[cryptography]==3.5.0
 bcrypt==4.2.1
-python-multipart==0.0.27
+python-multipart==0.0.31
 apscheduler==3.10.4
 python-nmap==0.7.1
 pyyaml==6.0.2