feat(demo): prepared sample request that is always viewable #71

Goal

Every visitor of the showcase can see a complete case – original, extracted values with sources, review and export – even while the model provider is unavailable. Context: showcase review 2026-09-27 rated the live demo 4/10 because a request stayed "In Verarbeitung"; root cause measured in #67: the Gemini API free tier answers about one call in three with 503 UNAVAILABLE (high demand), and on Vercel Hobby a job retry waits for a user action or the daily cron. Epic #19.

Acceptance criteria

• A seed step creates, per demo company, one synthetic sample request whose extraction comes from a recorded model response (no live call), stored like any other run
• The sample is clearly labelled in list and detail ("Vorbereitetes Beispiel – aufgezeichnete KI-Antwort") and its run records the model id as recorded
• The seed is idempotent: it creates a new sample only if no unreviewed sample exists
• The sample contains at least one uncertain and one missing field so the value of human review is visible
• A second sample per demo company is already approved and exported through the normal path (ERP mock), so a visitor sees review decision and ERP reference without any write action (review 2026-09-27: "Original, Extraktion, Prüfentscheidung und Exportbeleg")
• Runbook §6 names the step

Not part of this task

• Nightly reset, public guest access

Test plan

Security/Privacy affected?

Synthetic data only; recorded response is committed as a fixture.

Follow-ups from the same review: #74 (source view), #75 (case study), #76 (evidence), #77 (next action).