No error_for_status check on ERCOT HTTP responses

[haydenk]

Neither list_documents() nor download() in crates/downloader/src/ercot.rs calls .error_for_status() on the reqwest response. A 403, 404, or 500 from ERCOT is silently treated as a valid response.

• For list_documents() this produces a confusing JSON parse error
• For download() this writes an HTML error page to disk as a ZIP file, corrupting the ETL queue

Fix: Call .error_for_status()? on the response in both methods before reading the body.