feat(billing): add free credits for out-of-the-box model usage

[KurosawaGeeker]

Area

Billing

Problem

New users must configure their own provider credentials before they can complete a successful Run. This blocks the out-of-the-box experience.

Mosoo needs a renewable free-credit allowance so eligible users can try the product immediately.

Proposal

Give eligible users free credits that refresh on a defined schedule, such as monthly.

• Credits can be used with every model declared in Mosoo (all configured models), rather than a single free-tier model.
• Deduct credits according to each model's usage cost.
• Show the remaining balance, next refresh, and an actionable state when credits are exhausted.
• Keep existing user-provided provider credentials working unchanged.

Product decisions to resolve before implementation:

• Credit amount and refresh cadence.
• Eligibility, abuse controls, and behavior at exhaustion.
• Credit deduction rules across models with different prices.

Acceptance criteria:

• An eligible new user receives free credits without first adding a provider key.
• The allowance refreshes automatically on the configured schedule.
• Every model declared in Mosoo can consume the free-credit balance.
• Users can see their remaining credits and next refresh time.
• Exhausted or ineligible users receive a clear next action instead of a generic provider error.
• Existing configured-provider behavior remains compatible.
• Tests cover granting, refreshing, deducting, and exhausting free credits across declared models.

Contribution

I can provide product feedback.

Checklist

• I searched existing issues before opening this request.
• I described the problem before the proposed solution.
• I kept this request focused on one improvement.