#672 Promote test → main: free-tier card gate + instant backfill drain (chat#1796)

Promote #671 from test to main (prod).

• Free-tier card-on-file link — the Songstats gate now issues a Stripe mode:"setup" ($0, no subscription) checkout instead of the $99/mo subscription.
• Instant backfill drain — a historical job kicks songstatsBackfillWorkflow immediately (budget-gated, SKIP LOCKED; cron stays backstop). Verified on preview: enqueue → done in ~10s.

After this lands, a fresh prod agent key on a historical job returns the new $0 setup checkout link.

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Summary by cubic

Switches the Songstats gate to a free-tier card-on-file checkout and starts historical backfill immediately after enqueue to reduce wait time.

• New Features
  • Free-tier card gate: added createCardOnFileSession using Stripe Checkout mode: "setup" ($0, no subscription). Used when no customer or no default payment method.
  • Instant backfill drain: after enqueuing a historical job, call start from workflow/api to run songstatsBackfillWorkflow right away. Budget-gated and SKIP LOCKED; only triggers when items were enqueued.

Summary by CodeRabbit

Release Notes

• Improvements
  • Enhanced payment method collection process with a dedicated card setup flow for secure payment information storage.
  • Improved reliability of background data synchronization with better error handling to ensure uninterrupted data consistency.