feat(py): implement p0 scenarios (2/2) and http checks

[jsugg]

What

Adds the remaining Python P0 scenarios: login negative paths, file upload, iFrame editing, multiple windows, status codes, redirects, and broken images. Adds the Python HTTP/resource checks for basic auth, digest auth, direct download, and slow-resource timeout coverage. Updates catalog coverage, generated matrices, and Python stack docs.

Why

Phase 7 completes Python P0 coverage and adds the Python HTTP/resource slice requested for P7.3.

Verification

• rtk python3 tools/check-scenarios.py --write-matrix && rtk git diff --exit-code docs/scenario-matrix.md → passed.
• cd stacks/python-playwright && .venv/bin/ruff check . && .venv/bin/ruff format --check . && .venv/bin/mypy src tests → 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 && 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/p7-3-smoke --junitxml test-results/p7-3-smoke/junit.xml → passed (7 passed).
• cd stacks/python-playwright && THE_INTERNET_BASE_URL=http://localhost:7080 .venv/bin/pytest -m http --browser chromium --tracing retain-on-failure --screenshot only-on-failure --video retain-on-failure --output test-results/p7-3-http --junitxml test-results/p7-3-http/junit.xml → passed (6 passed).
• cd stacks/python-playwright && THE_INTERNET_BASE_URL=http://localhost:7080 .venv/bin/pytest --browser chromium --tracing retain-on-failure --screenshot only-on-failure --video retain-on-failure --output test-results/p7-3-chromium --junitxml test-results/p7-3-chromium/junit.xml → passed (24 passed).
• cd stacks/python-playwright && THE_INTERNET_BASE_URL=http://localhost:7080 .venv/bin/pytest --browser firefox -m 'not http and not not_ci and not flaky_demo' --tracing retain-on-failure --screenshot only-on-failure --video retain-on-failure --output test-results/p7-3-firefox --junitxml test-results/p7-3-firefox/junit.xml → passed (18 passed).
• cd stacks/python-playwright && THE_INTERNET_BASE_URL=http://localhost:7080 .venv/bin/pytest --browser webkit -m 'not http and not not_ci and not flaky_demo' --tracing retain-on-failure --screenshot only-on-failure --video retain-on-failure --output test-results/p7-3-webkit --junitxml test-results/p7-3-webkit/junit.xml → passed (18 passed).

Out of scope

Advanced P1/P2 Python ports stay reserved for later selective-port phases.