perf: enable ClickHouse async_insert to batch small inserts server-side

Diagnostics showed 3,200 tiny inserts per 5 minutes (processor_executions:
2,376 at 14 rows avg, logs: 803 at 5 rows avg), each creating a new part
and triggering MV aggregations + background merges. This was the root cause
of ~400m CPU usage at 3 tx/s.

async_insert=1 with 5s busy timeout lets ClickHouse buffer incoming inserts
and consolidate them into fewer, larger parts before writing to disk.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

deploy/clickhouse.yaml

@@ -182,6 +182,10 @@ data:
                 <max_block_size>8192</max_block_size>
                 <queue_max_wait_ms>1000</queue_max_wait_ms>
                 <max_execution_time>600</max_execution_time>
+                <!-- Buffer small inserts server-side before creating parts -->
+                <async_insert>1</async_insert>
+                <wait_for_async_insert>0</wait_for_async_insert>
+                <async_insert_busy_timeout_ms>5000</async_insert_busy_timeout_ms>
                 <!-- Disable parallel parse/format to reduce per-query memory -->
                 <input_format_parallel_parsing>0</input_format_parallel_parsing>
                 <output_format_parallel_formatting>0</output_format_parallel_formatting>