Modernize PointOfFocus for production deployment

[ibrahimjspy]

Summary

• Refactors the service into a testable Flask app factory with typed errors, resource limits, and correct HTTP status codes (404/405 no longer masked as 500).
• Upgrades runtime deps (Flask 3.1, OpenCV 4.13 headless, NumPy 2.x, requests, gunicorn) and hardens Docker with health checks, non-root user, and Gunicorn tuning.
• Adds pytest suite (33 tests, ~89% coverage), ruff linting, and GitHub Actions CI.
• Extends /focus with algorithm selection, configurable center bias, saliency score, optional crop box, and multipart uploads while keeping the core JSON contract stable.

Test plan

• CI passes on this PR (pytest + ruff)
• GET /health returns 200
• GET /focus?path=<local-image> returns focus + dimensions
• GET /focus?url=<image-url> works for internal image URLs
• POST /focus with multipart file returns 200
• Oversized image / tiny image return 413 / 400 with structured error codes
• Docker build and run: docker build -t saliency-focus . && docker run --rm -p 5000:5000 saliency-focus