docs(sdk): document billing.usageByClient

sdk.mdx

@@ -103,7 +103,7 @@
 
 - `outputUrl(job)` returns the headline URL: the one file, or the HLS/DASH manifest. It is `undefined` for data-only jobs, file sets, and non-complete jobs.
 - `output.files` is every file, for sequences, HLS segments, or a resolution ladder.
 - `jobData<T>(job)` reads `output.data` as `T | null` for probe, detection, and transcript jobs. Validate `T` yourself for untrusted input.
 - A `failed` job carries `JobError { code, message, detail, retryable }`.
 
 ```ts
@@ -133,7 +133,7 @@
 
 **Batches.** `batches.create({ jobs: [...] })` submits many jobs at once and returns `batchId`, `jobCount`, and `jobIds`. Wait on each id.
 
-**Billing.** `billing.state()` for balance and plan, `billing.usage({ start, end })` for spend, `billing.transactions({ page, limit })` for the ledger.
+**Billing.** `billing.state()` for balance and plan, `billing.usage({ start, end })` for spend, `billing.usageByClient({ days })` for spend grouped by the client that ran each job (dashboard, sdk, cli, mcp, n8n, or api), `billing.transactions({ page, limit })` for the ledger.
 
 **Webhooks.** `webhooks.create({ name, url, subscribedEvents })` registers an endpoint. Verify every delivery with the zero-dependency `verifyWebhookSignature` from `@rendobar/sdk/webhooks` (reads the `X-Rendobar-Signature` header, returns `Promise<boolean>`).
 
@@ -144,17 +144,17 @@
 if (!valid) return new Response("invalid signature", { status: 401 });
 ```
 
 **Realtime.** `realtime.subscribeJob(id, { onProgress, onStep, onComplete })` streams one job, `realtime.connect({...})` streams all org events.
 
 ```ts
 const sub = client.realtime.subscribeJob("job_abc", { onProgress: (e) => console.log(e.progress) });
 ```
 
 <Warning>
   Realtime needs session-cookie auth, not API keys. API-key users poll with `jobs.wait`.
 </Warning>
 
 Every response and param type is exported from `@rendobar/sdk` as a named `type`.
 
 ## Config and errors
 
@@ -184,7 +184,7 @@
 }
 ```
 
 Codes: `UNAUTHORIZED`, `FORBIDDEN`, `VALIDATION_ERROR`, `INSUFFICIENT_CREDITS`, `RATE_LIMITED`, `NOT_FOUND`, `CONFLICT`, `INTERNAL_ERROR`. The client auto-retries 429 (respecting `Retry-After`) and 5xx with backoff. Other 4xx throw immediately.
 
 ## See also