fix: resolve all linting errors, add tests, bump base image

Linting (ruff):
- cli.py: sort relative imports (I001)
- jobs.py: remove unused get_people import (F401), wrap long line (E501)
- executor.py: remove unused success variable (F841), wrap 4 long lines (E501)

Tests (24 passing):
- tests/test_config.py: config singleton defaults + env var overrides
- tests/test_upload_tracker.py: mark/filter/reset/summary logic
- tests/test_immich_api.py: filter_recent_assets date boundary cases
- tests/test_jobs.py: _resolve_strategy with LIMIT env var and fallbacks
- pyproject.toml: add [tool.pytest.ini_options] testpaths=["tests"] so
  pytest doesn't scan .venv in CI

Security:
- Dockerfile: bump CUDA base from 12.6.3 to 12.9.2 to pick up patched
  Ubuntu packages (fixes Dependabot low-severity alert)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -2,7 +2,7 @@
 #   amd64:  NVIDIA CUDA 12.6 (GPU acceleration when available, CPU fallback)
 #   arm64:  Plain Ubuntu (CPU-only, no CUDA on ARM)
 
-FROM --platform=$BUILDPLATFORM nvidia/cuda:12.6.3-cudnn-runtime-ubuntu22.04 AS base-amd64
+FROM --platform=$BUILDPLATFORM nvidia/cuda:12.9.2-cudnn-runtime-ubuntu22.04 AS base-amd64
 FROM --platform=$BUILDPLATFORM ubuntu:22.04 AS base-arm64
 
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