Add Airflow DAGs for existing pipelines

[kiyeonjeon21]

Priority: P1

Airflow dags/ directory is empty. Create sample DAGs orchestrating existing verified pipelines.

What to do

• dags/dbt_elt_pipeline.py — runs dbt seed, run, test via BashOperator
• dags/clickstream_monitor.py — checks ClickHouse row counts
• Follow Airflow best practices (proper default_args, doc_md, no top-level code)

Acceptance criteria

• All DAGs appear in Airflow UI at :8080 without import errors
• dbt_elt_pipeline DAG runs successfully end-to-end
• README documents which recipes must be running for each DAG