TASK-0000: ✨ Add durable ingestion billing outbox

[finnp]

Summary

• accept an Interloom billing context containing a stable ingestion ID, organization, and file or note attribution
• persist a pending billing event before each asynchronous LiteLLM completion, embedding, or transcription request and complete it with normalized token usage
• propagate billing identifiers into LiteLLM spend-log metadata for reconciliation
• deliver completed events at least once through a leased, retrying Postgres outbox using a service-authenticated callback
• preserve billing context through both simple and Hatchet ingestion workflows

Callback contract

Set R2R_BILLING_CALLBACK_URL and R2R_BILLING_CALLBACK_SERVICE_TOKEN. Delivery uses POST with the il-service-token header. Events are versioned with schema_version: 1 and deduplicated by billing_event_id on the receiver.

Verification

• pytest tests/unit/test_billing.py tests/unit/app/test_routes.py -q --no-cov (34 passed)
• ruff check core tests/unit/test_billing.py
• ruff format --check core tests/unit/test_billing.py
• scoped mypy with --follow-imports=skip (8 changed source files)

The repository-wide unit run reaches existing database-backed tests that require a local test Postgres; targeted route and billing suites are green.