feat(py): scaffold pytest-playwright stack

[jsugg]

What

Adds the Python Playwright stack under stacks/python-playwright with pytest, pytest-playwright, Ruff, mypy, the first smoke login scenario, and stack documentation. Extends the catalog checker and generated matrix to verify Python scenario IDs. Adds Python fast/full/nightly CI wiring and a Dependabot pip entry.

Why

Phase 7 starts the Python Playwright track while keeping its gate model aligned with the TypeScript Playwright stack per proposal §8 / R24.

Verification

• rtk python3 tools/check-scenarios.py --write-matrix && rtk git diff --exit-code docs/scenario-matrix.md → passed.
• rtk docker run --rm -v "$PWD:/repo:ro" -w /repo rhysd/actionlint:1.7.12@sha256:b1934ee5f1c509618f2508e6eb47ee0d3520686341fec936f3b79331f9315667 -color → passed.
• rtk npx --yes markdownlint-cli2@0.23.0 README.md stacks/python-playwright/README.md docs/scenario-matrix.md → passed.
• cd stacks/python-playwright && .venv/bin/ruff check . && .venv/bin/ruff format --check . && .venv/bin/mypy src tests → passed.
• cd stacks/python-playwright && THE_INTERNET_BASE_URL=http://localhost:7080 .venv/bin/pytest -m smoke --browser chromium --tracing retain-on-failure --screenshot only-on-failure --video retain-on-failure --output test-results/post-commit-smoke --junitxml test-results/post-commit-smoke/junit.xml → passed (1 passed).

Out of scope

Remaining Python P0 scenarios are reserved for P7.2 and P7.3.