Concurrency-safe subscription billing

Building a multi-tenant billing engine meant confronting a real race condition: two concurrent requests could both read the same usage quota before either wrote back, letting both through even when only one should've been allowed.

I moved usage tracking to a single atomic database update guarded by the remaining-quota condition, so the database enforces the invariant instead of application code racing against itself.

Plan changes are versioned as snapshots taken at renewal, so switching plans mid-period never retroactively corrupts what a workspace already consumed.

Quota, policy, and resource-lock violations return distinct, typed error responses instead of a generic failure, so clients can branch on what actually went wrong.