Add API coverage and diagnose E2E target-access failures

[larelgit]

Adds 15 API tests alongside the 12 UI scenarios, with smoke/regression selection and five offline fixture checks. PRs run smoke; main, scheduled and manual runs execute full regression. Dependencies and pytest/Ruff/Pyright settings now live in pyproject.toml.

The previous failed CI run received access-challenge pages and API redirect loops. Preflight diagnostics now expose those failures before the suites run; API requests stop at unexpected redirects, browser concurrency is reduced, URL overrides work under xdist, and account cleanup is validated. Persistent restrictions from the public target still fail CI.

Updates the README to describe production-style UI/API automation and adds an evidence-backed report for an existing upstream pytest-base-url/xdist defect.

Validation:

• Full local suite: 32 passed without reruns (15 API, 12 Chromium UI, 5 offline).
• Firefox UI: 12 passed; WebKit UI: 12 passed using temporary extracted Ubuntu libraries.
• Ruff lint/format, Pyright, actionlint and pip dependency checks passed.
• Local preflight checks correctly fail on redirect loops, HTTP 503 and HTML challenges.