feat(billing): generic usage submission webhook for marketplace billing providers

Problem

Marketplace partners that want to show real-time usage to their users in their own UI need PostHog to submit projected period usage on a daily cadence. Today this is hardcoded for Vercel. Generalize so any partner with Customer.billing_provider set gets routed through their MarketplaceBillingProviderClient.submit_usage().

What needs to happen

1. Daily cron for active customers with a billing_provider set.
2. Normalized usage payload — generic shape, lifted from Vercel's, that all providers receive:

   {
     "timestamp": "...",
     "eod": "...",
     "period": { "start": "...", "end": "..." },
     "billing": [ /* line items projected for current period */ ],
     "usage": [
       { "name": "Events", "type": "total", "units": "events", "dayValue": 0, "periodValue": 0 }
     ]
   }
   

3. Optional per provider — partners that don't need live usage can disable submission via their provider config; the cron skips them.
4. Retry semantics on partner-API outage — softer than invoices (a missed daily push isn't critical) but observable.

Reference

• Existing Vercel implementation: billing/billing_providers/clients/vercel.py:submit_usage
• Existing formatter: billing/billing_providers/formatters/vercel.py

Parent

Part of #57482.