Media Path Billing

[pragmatrix]

For more exact billing the support to push out billing and bill records through the media path needs to be supported.

[Copilot]

Pull Request Overview

This PR enables inband billing by routing billing records through the media path, adds a scheduling model for when to bill, and updates downstream services to specify billing schedules.

• Introduce service field and switch BillingRecords to media path in protocol.
• Add BillingSchedule enum and update ConversationOutput::billing_records for immediate vs. media-path billing.
• Update services and the audio-knife scheduler/dispatcher to tag and route billing records with schedules.

Reviewed Changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/protocol.rs	Add service field to BillingRecords and use OutputPath::Media.
src/context_switch.rs	Populate service in ServerEvent::BillingRecords.
services/playback/src/lib.rs	Emit per-frame billing with BillingSchedule::Media.
services/openai-dialog/src/lib.rs	Pass BillingSchedule::Now to billing calls.
services/azure/src/translate.rs	Include BillingSchedule::Now in billing calls.
services/azure/src/transcribe.rs	Include BillingSchedule::Now in billing calls.
services/azure/src/synthesize.rs	Include BillingSchedule::Now in billing calls.
core/src/lib.rs	Expose new billing_context module.
core/src/conversation.rs	Add BillingSchedule, update billing_records logic and Output::BillingRecords.
core/src/billing_context.rs	Introduce BillingContext for centralized billing.
audio-knife/src/main.rs	Wire billing_collector into ContextSwitch and dispatcher.
audio-knife/src/event_scheduler.rs	Refactor scheduler to handle media-path billing events.