Add reproducible Python quality checks and CI lint job

[AlexKitipov]

Motivation

• Make formatting, import ordering and static typing checks reproducible across environments by providing pinned tool configurations and CI enforcement.

Description

• Add pyproject.toml with Black, isort and mypy settings (Python target, excludes, mypy plugin and selective error suppression).
• Add .pre-commit-config.yaml with pinned hooks for black, isort, flake8, and mypy to enable local pre-commit enforcement.
• Add a dedicated lint job to .github/workflows/ci.yml that runs black --check, isort --check-only, flake8 and mypy before the existing backend/frontend jobs.
• Run isort/black and apply formatting/import fixes to several backend files so the new checks pass (notable files: app/api/v1/links.py, app/api/v1/analytics.py, app/api/v1/users.py, app/models/link.py, app/models/analytics.py, app/models/audit_log.py, app/config.py, app/database/base.py, migrations/versions/0001_initial_schema.py).

Testing

• Ran formatting and lint checks with python -m black --check app tests migrations && python -m isort --check-only app tests migrations && python -m flake8 app tests migrations --max-line-length=88 --extend-ignore=E203,W503 --exclude=.git,__pycache__,.mypy_cache,.pytest_cache,.venv,build,dist,node_modules && python -m mypy --config-file pyproject.toml and the checks passed after applying formatting and adjusting mypy configuration.
• Ran python -m pytest and all tests passed (26 passed).
• Attempted to run python -m pre_commit run --all-files after installing pre-commit, but installation was blocked in this environment due to the package index returning 403, so the pre-commit hooks could not be exercised end-to-end here.

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Codex Task