[Fix] Setup Sessions no longer open without their first turn

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote, in the web UI, or in Slack.

What changed

• Persist each web Fast Session's initial-turn payload inside the same advisory-locked transaction that creates or recovers the conversation.
• Reschedule deterministic retries while that durable initial turn remains pending, then reload it under the existing per-conversation turn lock before execution.
• Mark the initial turn complete only after successful execution so interrupted callbacks remain recoverable without rerunning completed turns.
• Preserve handled Fast failures as retryable kickoff failures after their user-visible error closeout is persisted.
• Add persistence, crash-window, completed-turn, and failed-retry regression coverage.

Why this change was made

Setup could commit a starter Session and complete onboarding before its first Fast turn ran in the Next.js post-response callback. If the process exited in that window, the deterministic retry found the existing Session and deliberately skipped the only launch opportunity.

Impact

Setup starter Sessions retain a recoverable first turn across process interruption. Retrying the same launch batch schedules pending work, while the durable completion marker and turn lock prevent completed first turns from running again.

[roomote-community]

No new code issues found. See task

• apps/web/src/trpc/commands/fast-sessions/index.ts:210 Do not complete the durable initial-turn marker when answerFastAgentQuestion has handled a failed attempt by returning an error closeout.

_{Reviewed ee026a5}