Extend usage-metric recording beyond operator-only visibility

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Problem: Existing usage-metric recording is scoped to giving the operator their own visibility — there's no per-customer view for a hosted model.

Area: ORB / Billing

Proposal: Extend the existing recording path to support a per-customer view, building on the durable-persistence work above.

Deliverables:

• Per-customer usage visibility, not just operator-wide.

[linear-code]

JSO-3576

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Re-scoping — the data layer already shipped: #7176/#7183 added the installation_id column and sumAiCostForTenantSince() aggregate to ai_usage_events. But nothing in src/api/routes.ts calls it — the aggregate is currently only exercised by test/unit/ai-usage-tenant.test.ts, no product code path uses it. Narrowing this issue to: expose sumAiCostForTenantSince() through a real API route/dashboard view. The ledger and aggregation work is already done; this is purely the surfacing layer now.