Add tenant column to ai_usage_events for centralized hosted billing across ORB + AMS

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Problem

ai_usage_events (src/db/schema.ts) already records AI usage/cost per event, but has no installation_id/tenant column — identity today is only recoverable indirectly via actor/route/metadata_json. A central hosted-billing aggregator (shared by both ORB and AMS once AMS's storage migration lands) needs a real tenant column to attribute usage without parsing free-form metadata.

Area

src/db/schema.ts (aiUsageEvents), src/db/repositories.ts (recordAiUsageEvent and its callers), a new migration.

Proposal

Add an installation_id (or tenant_id) column, backfill-nullable for existing self-host rows (self-host has no installation concept and should keep working unchanged), populated going forward by hosted containers at insert time.

Note this table double-purposes as a BYOK key-lifecycle audit log too (model prefixed byok:...) — the new column applies to both purposes; don't special-case one away.

Deliverables

• Migration adding the column
• recordAiUsageEvent accepts and writes it
• Existing self-host call sites unaffected (column nullable, no behavior change when absent)
• A basic aggregation query (e.g. sumAiCostForTenantSince) mirroring the existing sumAiEstimatedNeuronsSince/sumByokAiUsageForRepoSince pattern

Links & Resources

• Design: shared hosting control-plane for ORB + AMS SaaS (provisioning, secrets, billing, health) #7173 (parent — shared hosting control-plane)
• src/db/schema.ts:1289 (aiUsageEvents), src/db/repositories.ts:3556 (recordAiUsageEvent)

[linear-code]

JSO-4934