Billing: add cost and credit accounting safeguards

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Goal

Strengthen cost and credit accounting around paid external providers.

Scope

• ensure credits are deducted exactly once per successful/started processing job according to the intended policy
• make refunds/failure states explicit
• add admin-visible accounting for provider runs and failed attempts
• test race conditions and duplicate submissions
• document cost-control behavior for self-hosters

Why this matters

MelodAI calls paid APIs for separation, transcription, and LLM processing. Clear accounting protects maintainers and users from accidental overspend.