Include sunset_url in the 410 export-only response body #1938

The gap

docs/architecture/sunset.md lists this under Known gaps:

The 410 body carries no sunset_url.

Why it matters

While the hosted service is export-only, SunsetWriteBlockMiddleware answers every blocked write with 410 Gone and a detail sentence and nothing else. A client that hits that wall has no machine-readable way to reach the page explaining what happened — it would have to already know to call GET /health, which does return sunset_url. The two places a client learns the service is winding down should hand back the same URL.

The hosted data is purged on 18 Oct 2026, so anything that helps a user find the export page is worth doing soon.

Where

• surfsense_backend/app/sunset.py — the JSONResponse at the end of SunsetWriteBlockMiddleware.dispatch(), and the _DETAIL constant above it.
• sunset_url() already lives in that same module and is what GET /health reads, so the value is there for the taking. Don't add a second way to read SUNSET_URL.

What done looks like

• The 410 body carries sunset_url next to detail.
• It comes from sunset_url(), so a deployment that sets SUNSET_URL changes the health field and the 410 together.
• The "Refusing writes" section of docs/architecture/sunset.md quotes the refusal body — update that quote.
• Delete the Known gaps line in the same PR.

This is not a contract change: contract 4 covers GET /health only and says nothing about the 410 body, so you do not need sign-off from both sides.

How to verify

Tests for this middleware already exist in surfsense_backend/tests/unit/test_sunset_write_block.py. Add one that a blocked write returns 410 with sunset_url in the body, and one that a custom SUNSET_URL shows up there. Write the failing test first.

cd surfsense_backend
uv run pytest -m unit -k sunset
uv run ruff check .
python ../scripts/check_docs.py

Before you start

Read CONTRIBUTING.md and the sunset doc. Open your PR against dev, not main. Comment here to claim this.