Allow users to purchase additional AI credits without upgrading their plan

What problem does this solve?

Our AI features (available in Mixpost) are tied to a fixed monthly credit allowance per subscription tier. Users who occasionally need more credits — for example during a busy month or a one-off campaign — currently have no way to get extra credits without upgrading to a significantly more expensive plan. This creates friction and dissatisfaction: users end up paying for a much larger plan just to cover a temporary spike in AI usage, or they simply stop using the AI features once their credits run out, reducing engagement with a core part of the product.

Describe the solution

Introduce the ability for users to purchase additional AI credits as a one-time or recurring top-up, independent of their current subscription tier. This could work as follows:

• A "Buy more credits" option in the billing/AI usage section of the dashboard.
• Credit packs available in fixed sizes (e.g. 100 / 500 / 1000 credits) purchasable via the existing billing/payment integration.
• Purchased credits are added on top of the plan's existing monthly allowance and either roll over or expire after a defined period (configurable).
• Clear visibility of remaining credits (plan-included vs. purchased) in the usage dashboard.
  This gives users flexibility to scale their AI usage as needed without forcing an upgrade, and creates an additional revenue stream for SaaS operators built on Mixpost.

Alternatives considered

• Forcing an upgrade to a higher plan: current behavior — leads to poor user experience and lost revenue from users unwilling to upgrade for a temporary need.
• Uncapped/unlimited AI usage per plan: not feasible, since AI usage is directly tied to real provider costs (e.g. OpenAI/Anthropic API costs) and needs to remain controlled.
• Manual credit grants by support/admin: possible as a stopgap, but doesn't scale and requires manual intervention for every request.