Fix starting a second case after case completion

Six-municipality Chrome acceptance reproduced this consistently in meetings 10–15. After ending the first case, clicking either “Start neste sak” or the case card’s start action navigates to the OngoingCase tab, but it renders “Venter på neste sak”; reload leaves the target case not_started. No actionable error is shown. In some runs the first case also required retries and Turbo reported that form responses must redirect. Trace the asynchronous StartCaseJob/command ID/run sequence and redirect/broadcast behavior after an earlier case has ended. Starting the next case must persist canonical ongoing state exactly once, return a valid top-level Turbo navigation, and update secretary, politician, big-screen, and public views without reload. Add sequential multi-case browser/request coverage.