feat(billing): add cloud usage billing foundation

[pandemicsyn]

Summary

• Add the Milestone 1 foundation for usage-based cloud billing: typed container lifecycle RPC contracts, persisted heartbeat/stop retry state, and a new container-usage-meter Worker with a durable SQLite failover buffer.
• Add immutable, admin-managed cloud_billing_sku products with exact cents-per-second rates, one-way disablement, and primary-backed credit-manager authorization. The restricted admin/API surface exposes create, list, and one-way disable operations only.
• Add a Cloud Billing SKUs admin page for creating, listing, previewing, and disabling SKUs, including exact decimal price previews and explicit loading, error, permission, validation, and pending states.
• Validate recordStart against the SKU catalog and durably serialize accepted or rejected start decisions so retries remain consistent across SKU disablement and PostgreSQL outages.
• Add the PostgreSQL interval schema, Durable Object SQLite migrations, Worker configuration, local-service registration, and generated bindings needed for the foundation. Producer wiring, PostgreSQL interval ingestion, charging, and budget enforcement remain follow-on milestones.

Verification

• Opened /admin/cloud-billing-skus through the local fake-admin flow and verified the page renders in the existing admin shell with the Financial navigation entry, immutable-SKU guidance, catalog card, and permission-gated create action.
• Manual create/disable flow was not exercised because the long-lived local PostgreSQL database had already recorded an earlier uncommitted migration with the same sequence; the final schema was instead validated against fresh databases.
• Additional manual verification details:

Visual Changes

Before	After
No Cloud Billing SKUs admin surface	New Cloud Billing SKUs catalog and immutable SKU creation workflow; screenshot to attach before review

Reviewer Notes

• This PR delivers Milestone 1, Foundation.
• Milestone 1 scope is durable contracts and capture semantics only: no producer is wired, and the meter does not yet apply intervals to PostgreSQL, drain/replay buffered mutations, reconcile stale intervals, or reclaim Durable Object rows.
• start_admissions (pending | accepted | rejected) and pending_usage_mutations intentionally remain append-only in this PR. Milestone 2 owns safe cleanup after PostgreSQL confirmation: retain pending admissions through validation retries, retain accepted admissions while dependent buffered mutations exist, retain rejected admissions through the idempotency window, and expose TTL/backlog metrics and alerts.
• Milestone 2, Meter and Shadow: wire Cloud Agent with an explicit SKU, add the PostgreSQL hot path, failover replay and reconciliation, then shadow-record usage and estimated cost without debiting credits.
• Milestone 3, Charge and Enforce: create auditable usage charges, debit the shared wallet idempotently, return budget verdicts on heartbeats, and pause containers at the configured balance floor.
• Focus review on the restricted SKU mutation surface, start-admission idempotency across PostgreSQL outages, lifecycle retry/fencing behavior, and the primary-backed creditManagerProcedure authorization change.
• Review this PR as a Foundation boundary rather than a partially deployed meter: there is deliberately no production producer traffic until Milestone 2 supplies PostgreSQL application, drain/reconciliation, and admission/mutation reclamation together.