chore: modernize Python tooling (uv, PEP 621, semantic-release, src layout)

[salman2013]

Summary

• Phase 1: Migrate package metadata to pyproject.toml (PEP 621) — removed setup.py, added setuptools-scm for dynamic versioning, coverage config, importlib.metadata for __version__
• Phase 2: Switch dependency management from pip-tools/requirements/*.txt to uv + uv.lock with [dependency-groups]; update tox.ini to use tox-uv/uv-venv-lock-runner; update CI to use astral-sh/setup-uv
• Phase 3: Add python-semantic-release workflow for automated PyPI publishing via OIDC trusted publisher; add commitlint workflow to enforce conventional commits
• Phase 4: Migrate to src/ layout — moved image_explorer/ to src/image_explorer/

Test plan

• uv run tox -e quality — pylint 9.45/10, passes
• uv run tox -e py312-django42 — 6/6 tests passed
• uv run tox -e py312-django52 — 6/6 tests passed
• Configure OIDC trusted publisher on PyPI for openedx/xblock-image-explorer (publisher: GitHub Actions, workflow: release.yml)

🤖 Generated with Claude Code

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