feat(usage): derive compute-usage rollups and prune the raw compute log (RIG-2872)

[rigel-mintaka]

The compute-usage event log from the previous PR gets the rest of the usage-store contract: derived rollups, a retention policy, and an in-memory reference checked by the same contract suite as Postgres.

Changes

• Migration 0005_compute_usage_rollups.sql adds hourly and daily rollups keyed by tenant, bucket, owner, and agent (active_ms, intervals), plus a compute prune horizon. Tenant RLS and grants match the token rollups.
• An AFTER INSERT trigger on compute_usage_events rolls up each end event. Every end-writing path (release, runner sweep, re-point, orphan sweep) is covered in the same transaction. Duration is split across UTC hour and day buckets; an interval counts once, in its starting bucket. Open intervals contribute nothing.
• RebuildComputeUsageRollups recomputes rollups from the prune horizon using the same clipping rule as the trigger.
• PruneComputeUsageBefore deletes both events of intervals that ended before the cutoff day, never an open start, and keeps rollups. The daily RetentionSweeper prunes token and compute logs with the same window.
• usage.Store gains ComputeUsageSeries, RebuildComputeUsageRollups, and PruneComputeUsageBefore. The memory backend is the reference implementation.

Verification

• Contract suite on memory and Postgres: bucket splits across hour and day, open-interval exclusion, rebuild equals incremental (including across a prune horizon and many crossing intervals), prune keeps rollups and open starts, tenant isolation.
• pgtest race: ./internal/usage, ./internal/store, ./internal/runnerhub, ./server, ./cmd/compass-server all ok. sqlc drift, migration gate, nilaway, and golangci-lint pass.

Refs RIG-2872

Co-authored-by: Matt Wilkinson matt@rigel.build

[linear-code]

RIG-2872

[github-actions]

Compass engineering docs preview: https://compass-server-rig-2872-comp-ueya.compass-eng-docs.pages.dev

Deployed from compass-server/rig-2872-compute-rollups at 68cc4c1.

Changed pages:

• docs/concepts/tokens-and-billing.md

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