Workflow-agnostic inbound dispatcher.

All providers can point a single webhook at this endpoint instead of one URL per workflow. The dispatcher resolves the org from the webhook's account_id and the workflow from the called number's inbound_workflow_id. This is what configure_inbound writes into each provider's resource so per-workflow webhook bookkeeping disappears.

Provider-specific signature/timestamp headers are not enumerated here — each provider's verify_inbound_signature reads its own headers from the dict, so adding a new provider doesn't require changes to this route.