Replace Vercel cron with Modal completion callback

[Yamamoto789]

The music hardening branch currently uses Vercel cron to resume durable analysis and cue-generation jobs. The team wants to avoid the Vercel Pro cron requirement.

Proposed change:

• Keep the existing durable Modal job IDs, leases, retries, and credit lifecycle.
• When Modal analysis finishes, it should POST a signed completion callback to ShowCrafter.
• ShowCrafter should validate the callback, complete the analysis, and resume cue generation.
• Keep the existing admin reconciliation route as a fallback for missed callbacks.
• Remove the minute-level Vercel cron dependency.

The implementation should not revert to synchronous analysis or rely on the browser remaining open.

Acceptance:

• analysis finishes and resumes cues without cron
• duplicate callbacks are safe
• failed callbacks have bounded retry/recovery
• existing tests remain green

[linear]

FIR-175