Add tenant-facing AI-usage/cost endpoint (sumAiCostForTenantSince has zero real callers) — unit decided: normalized compute-units

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Problem

sumAiCostForTenantSince (src/db/repositories.ts ~3678, added via #7176 for "centralized hosted billing") has zero real callers outside its own test file. Its fleet-wide sibling listAiCostByTenantSince is consumed only by src/services/operator-dashboard.ts, gated to the operator role — no ORB or AMS hosted tenant can see their own usage/spend anywhere today.

Area

src/api/routes.ts, src/services/.

Proposal

Unit decided (2026-07-21): normalized compute-units, mirroring packages/loopover-engine/src/tenant-quota.ts's TenantQuota.computeUnits shape — not raw costUsd. Add a tenant-facing usage/spend route calling sumAiCostForTenantSince, converting the raw cost figure into the same compute-unit representation tenant-quota.ts already uses, scoped to the requesting session's own installation the same way /v1/app/maintainer-dashboard (src/api/routes.ts ~1596-1619) already scopes its data.

⚠️ Required pattern. Reuse /v1/app/maintainer-dashboard's exact tenant-scoping call (loadControlPanelRoleSummary or whatever helper it calls — read that route first). A PR that invents a second scoping mechanism instead of reusing that one does not satisfy this issue, and a PR that exposes raw costUsd instead of the normalized compute-unit figure does not satisfy this issue either.

Deliverables

• A tenant-scoped usage/spend route exposing normalized compute-units (not raw cost), reusing the maintainer-dashboard's exact scoping pattern.

Test Coverage Requirements

src/** — standard 99%+ Codecov patch target.

Resources

• src/db/repositories.ts (sumAiCostForTenantSince, listAiCostByTenantSince)
• src/api/routes.ts ~1596-1619 (/v1/app/maintainer-dashboard, the required scoping pattern)
• packages/loopover-engine/src/tenant-quota.ts (the required unit shape)

Boundaries

Read-only exposure of an already-computed figure, converted to the decided unit — no changes to how cost itself is tracked or computed.

Re-locked to maintainer-only on 2026-07-21 — see the issue comment for why (tenant billing/usage-exposure surface, not purely mechanical). This line is left from before that correction; do not treat it as current guidance.

[linear-code]

JSO-5410

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Unblocked (2026-07-21): unit decided — normalized compute-unit abstraction (mirroring AMS's TenantQuota.computeUnits shape), not raw costUsd. Ready to implement.