chore(deps): update dependency pypdf to v6.15.0 [security]

Author: renovate[bot]

backend/requirements.txt

@@ -30,7 +30,7 @@ pydantic==2.12.5
 PyJWT==2.13.0
 pymongo==4.16.0
 PyNaCl==1.6.2
-pypdf==6.14.2
+pypdf==6.15.0
 pytest==9.0.3
 python-dateutil==2.9.0.post0
 python-dotenv==1.2.2