Start an export of the project's data to object storage (owner/admin)

Queues an asynchronous export. ClickHouse writes the data DIRECTLY to object storage — no bytes pass through this API — so the response is a job you poll, not a file. Each table produces a data object plus a <table>.schema.sql sidecar holding its CREATE TABLE (the only thing that carries ENGINE, ORDER BY, PARTITION BY, TTL and codecs; no data format does). Format native (default) is lossless. Format parquet is for interop and is lossy at two edges: Enum8/Enum16 keep their LABELS but not their ordinals (round-tripping into a differently-ordered Enum rewrites the data), and LowCardinality(T) returns as plain T. A table holding a type Parquet cannot represent (e.g. AggregateFunction) fails the job with an error naming the column. A ReplacingMergeTree is exported UN-MERGED — the faithful dump of what is stored. The produced objects are transfer scratch: they are deleted after 7 days, and each download link is signed for 15 minutes.