Per-account quotas exist as settings with no limit by default: the owner sets repositories and apps from Settings and the API, audited, nothing enforced yet #2092

Part 1 of #973 (a person's ask, parked twice). Both tries failed because default limits broke unrelated suites (test/fleet-graph.test.ts and test/studio-changes.test.ts create many repositories). So this part enforces nothing.

Done looks like

• [ ] src/core/quotas.ts reads and writes per-account limits for repositories and apps. The default for each is no limit (null), and there is room for #918's and #919's keys.
• [ ] The admin settings page and PATCH /api/v1/admin/settings set and clear each limit. A value must be a positive integer or empty, and anything else is a 422. Every change is recorded in the audit.
• [ ] Test test/quotas.test.ts: limits start unset, an admin sets one and clears it, a non-admin is refused, and a bad value is a 422.
• [ ] No create path changes. bun test test/fleet-graph.test.ts test/studio-changes.test.ts still passes.
• [ ] Don't rebuild the repo, run or repo_crew tables, because the steward's triggers live on them.

Suggested: scout.