Automate docs screenshots with Playwright

[deanleigh]

The UpDoc docs are text-heavy. A visual tool deserves visual documentation, but manually captured screenshots rot fast — every UI tweak invalidates them. Playwright can drive the backoffice through documented flows and capture screenshots at predetermined moments, regenerated on demand.

Groundwork already done: planning/DOCS_SCREENSHOT_AUTOMATION.md (merged in #18) captures the high-level idea. This issue is for the detailed design + implementation.

Why Playwright fits:

• Authentication already solved (tests/e2e/auth.setup.ts, Playwright API user, .env credentials)
• Known test content exists (Winchester PDFs, Group Tours tree)
• Screenshots regenerate from actual flows — stay in sync with reality
• One command rebuilds all docs imagery

Proposed first pass:

• Pick one docs page (candidate: updating-blueprints.md — small scope, proves the pattern)
• New folder tests/docs-screenshots/ kept separate from tests/e2e/
• Screenshots saved to docs/src/assets/screenshots/<page>/
• Helper wrapping page.screenshot() with fixed 1440×900 viewport, consistent output paths
• No separate Playwright config yet — prove the approach first, extract framework later

Detailed design: planning/DOCS_SCREENSHOT_AUTOMATION_SPRINT.md (follow-up) will cover:

• Test site prerequisites (running site, seeded workflow, clean state)
• Viewport / theme / annotation decisions
• Per-page screenshot spec structure
• How screenshots are referenced from markdown
• Review workflow (diffing PNGs in PRs)
• Generalisation path after first page works

Out of scope (for this issue):

• Video / GIF captures — future issue
• Localised docs — future
• Visual regression testing — this is artefact generation, not assertion