Allow self-hosted instances to reward Fluxer donations with Premium

Self-hosted instances can configure their own Premium billing, but there is currently no built-in way to direct Premium users toward supporting the upstream Fluxer project instead. Operators who do not want to collect Premium revenue themselves must either handle this manually or build their own custom flow.

Allow self-hosted instances to optionally use Fluxer donations as a way to grant Premium. A user could start from the existing Premium flow in the client, be directed to https://fluxer.app/donate, and after a successful donation have Premium granted on the self-hosted instance. This should be optional, so operators can still use their own Stripe billing if they prefer.

The main benefit would be giving self-hosted communities a simple way to offer Premium while directing financial support back to the upstream Fluxer project. Ideally, the process could be automated so users do not need to submit donation receipts manually.