Bump PyJWT to 2.15.1 in backend

[katsugtgz]

Updates PyJWT to address GHSA-ffc3-869f-jxw9, GHSA-9j54-fg26-wv3r, GHSA-p4g4-x82p-q773, GHSA-w2cx-738m-mc7w, GHSA-r6x4-923q-g947, GHSA-9v7f-9g4p-ffgj, GHSA-42vr-xj54-vc7v and PYSEC-2026-4141.

Evidence:

• backend/requirements.txt and backend/uv.lock referenced PyJWT==2.13.0
• OSV query for PyJWT@2.13.0 reports the IDs above (critical: GHSA-ffc3-869f-jxw9)
• updated version: 2.15.1; OSV query for PyJWT@2.15.1 reports no advisories

Validation:

• uv lock --check passes after the edit (base branch passes it too, lock was in sync)
• installed from the lock with uv sync --frozen --all-extras, then ran the documented backend suite python -m pytest: 473 passed, 1 skipped
• scope: backend/requirements.txt and only the pyjwt entries in backend/uv.lock