[codex] Clean repository for local Yandex Music product

[karnaksp]

Product Value

This PR turns the repository surface into the current local Yandex Music self-analytics product only. It removes the old streaming demo and makes the public README/Pages experience Russian-first, visual, and runnable from the local product path.

Changes

• Replace README with a Russian product entrypoint: value proposition, local run, commands, architecture, privacy, and dashboard screenshots.
• Add tracked dashboard demonstration screenshots for Story, Atlas, and Actions from the local sample run.
• Remove legacy Eventsim/Kafka/Spark/Airflow/GCP/Terraform files, old images, seed data, and obsolete scripts.
• Keep dbt focused on the DuckDB yamusic models and add current orphan-key singular tests.
• Rebuild scripts/build_pages_site.py into a Russian GitHub Pages site with a modern hero, sticky navigation, dashboard preview, responsive layout, copied assets, image rendering, and favicon.
• Tighten scripts/validate_yamusic_local.py so old demo artifacts and stale English Pages markers cannot silently return.

Validation

• make test
• make pages-site
• python scripts/validate_yamusic_local.py
• Browser QA against local Pages server: Russian desktop/mobile layout, no horizontal overflow, no broken dashboard images, dashboard page renders 3 screenshots.