Clarify Streamify fork contribution

[karnaksp]

Summary

• add an explicit "Мой вклад в этом fork" section to README and data quality docs
• clarify that the added layer is dbt/Airflow data-quality verification around the existing streaming pipeline
• keep wording focused on project purpose and engineering value

Validation

• python3 scripts/validate_dbt_quality.py
• python3 -m compileall -q airflow/dags spark_streaming scripts
• cd airflow && GCP_PROJECT_ID=dummy GCP_GCS_BUCKET=dummy docker compose config --quiet
• cd kafka && docker compose config --quiet
• git diff --check
• rg portfolio/showcase terms in README/docs/scripts/.github/airflow/dbt