refactor(airflow): move DAGs to airflow/dags and update paths

- Why:
  - keep Airflow artifacts under a single airflow/ directory
  - align repository layout with intended project structure
- What:
  - move dags/ to airflow/dags/ and update compose mounts
  - make SQL root resolution work in container and local runs
  - update DAG path references in README, AGENTS, ARCHITECTURE, and plans
  - remove tracked Python cache artifacts from old DAG location
- Check:
  - airflow dags list
  - airflow dags list-import-errors
  - e2e success: ddl_init, kafka_load(limit=50), etl_pipeline

docker-compose.yml

@@ -108,7 +108,7 @@ services:
     ports:
       - "8080:8080"
     volumes:
-      - ./dags:/opt/airflow/dags
+      - ./airflow/dags:/opt/airflow/dags
       - ./sql:/opt/airflow/sql:ro
       - ./data:/opt/airflow/data
     networks:
@@ -133,7 +133,7 @@ services:
         airflow scheduler
       "
     volumes:
-      - ./dags:/opt/airflow/dags
+      - ./airflow/dags:/opt/airflow/dags
       - ./sql:/opt/airflow/sql:ro
       - ./data:/opt/airflow/data
     networks:
@@ -155,7 +155,7 @@ services:
     environment:
       <<: *airflow-default-env
     volumes:
-      - ./dags:/opt/airflow/dags
+      - ./airflow/dags:/opt/airflow/dags
       - ./sql:/opt/airflow/sql:ro
       - ./data:/opt/airflow/data
     networks: