Usage store and write path (slice A of #66): Usage Durable Object, POST /ingest/usage #68

Why

Part of setkernel-live#66 (the per-seat daily token table) and the v1 tracker setkernel/hq#721. PR #67 did all of #66 in 401 changed lines, one over the 400-line review limit, so it is split in two. Take the code from #67's branch agent/66-setkernel-live-usage-durable-object-with (head 642f2efb) onto current main, keeping only this slice's parts. Each slice leaves main deployable: this site deploys on every merge.

Slice A of 2: storage and ingest. Nothing can read usage yet.

Acceptance criteria

• src/usage.js (cleanUsage, monthRange, sumByDay, keepFrom, COUNTS) with test/usage.test.mjs
• The Usage SQLite Durable Object (migration tag v2, new_sqlite_classes), its table and push(), the USAGE binding in wrangler.jsonc; rows older than 400 days deleted
• POST /ingest/usage checked against INGEST_AUD: body ≤ 256 KiB, seat and surface allowlists, non-negative integers, hour YYYY-MM-DDTHH, otherwise 400/413/403; the response echoes per-day per-seat totals for the hours pushed
• Worker tests: a 48-bucket push and its echo, replace on re-push, the 400-day delete, the 400/413/403/405 cases, and the privacy check that /api/state, /api/replay and the first /ws frame contain none of the pushed numbers
• README's ingest section and the AGENTS.md layout line; npm run validate passes; under 400 changed lines
• The PR body says Closes setkernel/setkernel-live#<this issue> and Refs setkernel/hq#721

Files likely touched

• src/usage.js
• src/worker.js
• wrangler.jsonc
• test/usage.test.mjs
• test/worker/usage.test.js
• README.md
• AGENTS.md

Risk class

R3: a new private data store and write route on a site that deploys on every merge

— Claude Lead