Add verified PULSE showcase across pages

[TFT444]

What changed

• add a curated PULSE ZERO showcase record with exactly 20 ranked concepts, explicit winner and runner-up, and the verified run_ec58590cf52e provider evidence
• serve the saved Wan PNG and MP4 through stable local showcase routes with correct media types
• load the same curated prior-live run across every showcase page while keeping current operation labelled DEMO MODE: No sponsor calls
• add responsive cross-page showcase presentation and focused integrity, truth-labelling, route, and content-type tests

Why

The urgent judge-facing experience needed one consistent, genuine saved PULSE run across the platform and supporting pages without triggering new sponsor calls or presenting prior-live media as current activity.

Validation

• python -m ruff check .
• python -m pytest -q (187 passed)
• node --check src/sieve/web/app.js
• git diff --check

Scope

Only the five intended implementation/test files are committed. docs/sieve-hero.svg, .env, secrets, caches, and internal AI/skill files are excluded.