chore(deps): refresh backend dependencies

[RyleaStark]

Stack

• chore(deps): refresh frontend dependencies #1 — frontend dependency and npm security refresh
• This PR — backend runtime dependency refresh
• PR ci(deps): automate dependency maintenance #3 — dependency automation, security gates, and toolchain pinning
• PR #4 — pre-release minor-version audit and any required application changes
• PR chore: harden minor release readiness #5 — version bump and GHCR/Docker Hub publication

Merge after #1. This PR is intentionally limited to backend runtime dependency pins.

Summary

Updates every direct backend runtime dependency:

Package	From	To
FastAPI	0.110.0	0.141.1
Uvicorn	0.27.1	0.52.1
grpcio	1.60.0	1.83.0
protobuf	4.25.1	7.35.1
httpx	0.26.0	0.28.1
python-dotenv	1.0.1	1.2.2
pydantic-settings	2.2.1	2.14.2
websockets	15.0.1	17.0.1

Security

• clears the currently reported Python Dependabot findings
• pip-audit -r backend/requirements.txt reports no known vulnerabilities
• pip check reports no broken requirements

Verification

• Ruff — passed
• pytest — 92 passed
• FastAPI application import — 26 routes loaded
• clean Python 3.11.15 environment

Follow-up audit items

The updated stack emits non-failing Starlette deprecation warnings around HTTP_422_UNPROCESSABLE_ENTITY and the TestClient/httpx compatibility bridge. These are deferred to the pre-release audit PR rather than mixing application changes into this dependency-only layer.