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.

tests/conftest.py

@@ -0,0 +1,17 @@
+import os
+import sys
+
+_TESTS_DIR = os.path.dirname(os.path.abspath(__file__))
+_REPO_ROOT = os.path.dirname(_TESTS_DIR)
+
+# app/*.py open their JSON config files with plain relative paths, so the
+# process cwd must be the directory the app modules live in — matches how
+# the Dockerfile runs them (WORKDIR /app).
+_APP_DIR = os.path.join(_REPO_ROOT, "app")
+if not os.path.isfile(os.path.join(_APP_DIR, "gpu_checker.py")):
+    _APP_DIR = _REPO_ROOT  # container test stage: code copied flat next to tests/
+
+if _APP_DIR not in sys.path:
+    sys.path.insert(0, _APP_DIR)
+
+os.chdir(_APP_DIR)