Add Gitea Actions CI/CD pipeline

Bring nvidia-stock-bot up to the same CI/CD maturity as adguard-cidre:
pytest suite (19 tests), multi-stage Dockerfile with a dedicated test
stage, pinned requests dependency, Gitea Actions workflow (build, smoke
test, unit tests, deprecation check, Trivy critical/high scan, version
bump + registry publish + release), and a Renovate config for automated
dependency updates.

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.13-alpine
+FROM python:3.13.15-alpine AS base
 
 RUN apk add --no-cache ca-certificates
 
@@ -9,4 +9,15 @@ COPY /app/ /app/
 
 RUN pip install --no-cache-dir -r requirements.txt
 
-CMD ["python", "main.py"]
+FROM base AS test
+
+RUN pip install --no-cache-dir pytest==9.1.1
+
+COPY pytest.ini /app/pytest.ini
+COPY /tests/ /app/tests/
+
+CMD ["pytest", "-v"]
+
+FROM base
+
+CMD ["python", "main.py"]