fix: add early quota warning (#2986)

## Description

- Add banners when plans near fair use limits
- Automated email alerts to our support when nearing fair use limits

packages/lib/server-only/rate-limit/get-quota-alert-kind.ts

@@ -0,0 +1,40 @@
+export const QUOTA_WARNING_THRESHOLD = 0.8;
+
+export type QuotaAlertKind = 'quota' | 'quotaNearing';
+
+type GetQuotaAlertKindOptions = {
+  previousCount: number;
+  newCount: number;
+  quota: number;
+};
+
+/**
+ * Determines whether the request that moved the counter from `previousCount` to
+ * `newCount` crossed an alert threshold.
+ *
+ * - 'quota': this request reached (or jumped past) the monthly quota.
+ * - 'quotaNearing': this request reached the warning threshold (80% of quota).
+ * - null: no threshold crossed by this request.
+ *
+ * Precondition: callers must handle `quota === null` (unlimited) and `quota === 0`
+ * (blocked) before calling — this function assumes a positive quota.
+ */
+export const getQuotaAlertKind = (opts: GetQuotaAlertKindOptions): QuotaAlertKind | null => {
+  const { previousCount, newCount, quota } = opts;
+
+  if (newCount >= quota) {
+    // Only the single request that reached the quota boundary should alert. If the
+    // same request also skipped past the warning threshold, the quota alert
+    // supersedes the warning.
+    return previousCount < quota ? 'quota' : null;
+  }
+
+  // From here newCount < quota, so for tiny quotas (1-4) where the rounded-up
+  // warning threshold equals the quota itself, the warning can never fire — the
+  // exhausting request is handled by the quota branch above.
+  const warningCount = Math.ceil(quota * QUOTA_WARNING_THRESHOLD);
+
+  const didCrossWarning = newCount >= warningCount && previousCount < warningCount;
+
+  return didCrossWarning ? 'quotaNearing' : null;
+};