Run the pre-send checks for a broadcast

The review list the dashboard shows before sending: sender verification, subject, preview text, content, audience size, email size against Gmail's 102 KB clipping limit, postal address, merge tags without data or fallback, empty or shortened links, missing image descriptions, leftover placeholder text, spam-prone wording, the free trial's own-address rule, and whether a test was sent. Each check has a level (ok, warn, or block); ready is false while any check blocks. Scheduling runs the same checks.