fix(airflow): simplify kafka_load params and align docs

- Why:

  - For DE task we only need full ingest or limit-based sample.

  - load_* and full_load params were redundant and unclear in current flow.

- What:

  - Remove full_load and load_* params from kafka_load DAG contract.

  - Simplify kafka helpers (validate/check files) to fixed 4-stream ingest.

  - Sync AGENTS, README, runbook, architecture and airflow plan docs.

- Check:

  - python3 -m py_compile dags/kafka_load_dag.py dags/utils/kafka_helpers.py

  - Airflow smoke/full runs: ddl_init -> kafka_load -> etl_pipeline (all success).

  - Legacy path: make data && make transform (success).

docs/ARCHITECTURE.md

@@ -615,7 +615,7 @@ INSERT INTO dm.daily_traffic SELECT * FROM dm.v_daily_traffic;
 - Загрузка данных из `data/*.jsonl` в Kafka через `kafka-python`
 - TaskGroup `precheck`: проверка Kafka, файлов, параметров
 - TaskGroup `ingest`: создание топиков → параллельная загрузка 4 потоков → проверка
-- Параметры: `limit` (0 = все), `reset_topics`, `load_*` (выбор потоков)
+- Параметры: `limit` (0 = все), `reset_topics`
 
 **Подключение к ClickHouse:**
 - Connection: `clickhouse_default`