Adds a Preview action next to eligible partial/final CSV artifacts on
the job detail page. Reads at most 64 KiB and 30 rows via a coordinator-
owned blob open, verifying job ownership and the same downloadable rule
as the existing download proxy so an artifact ID from another job is
never disclosed. Non-CSV and malformed/empty content fail safely with a
sanitized message instead of being rendered as text; all cell values go
through html/template escaping.
