Audit and validate the ClickHouse backend

[jamessewell]

If you're a ClickHouse expert, we'd love your help reviewing our ClickHouse driver.

What this driver does

The ClickHouse driver (backends/clickhouse/) is a standalone backend implementation using the official clickhouse-go/v2 driver.

It implements the Driver interface:

• Exec — splits multi-statement SQL with pgsplit and executes each statement
• Query — runs a query, allocates generic scan targets from column types, and counts rows
• Insert — bulk inserts using ClickHouse's PrepareBatch API (append rows, then send)
• Update — delegates to Insert (relies on ReplacingMergeTree for deduplication)
• CaptureConfig — captures version() and key settings from system.settings (max_threads, max_memory_usage, max_block_size, max_insert_threads)

Connection pool: MaxOpenConns=20, MaxIdleConns=5, 30m lifetime. Pings on connect.