Implement gigabyte-month for storage instead of current system

[riderx]

Describe the bug/issue

The goal is to fit the same as Cloudflare, and invoice users on the same system.

This will make the chart always go up but allow users to go above their limit for a day without a hard stop

[TorichanCapgo]

Nice direction — aligning billing with GB-month is a strong UX + finance improvement 📈\n\nTo move this from idea to implementation, I suggest we lock these acceptance rules:\n- Sampling window (hourly vs daily)\n- Formula and units (GB-hour aggregated into GB-month)\n- Rounding policy for invoice line items\n- Grace behavior when usage spikes above plan for short periods\n\nIf helpful, I can draft a concrete implementation plan next (DB model + aggregation job + invoice preview changes) so this can be split into small PRs.