Validate the job envelope and make Retry poll the batch's job

useJob now takes the job kind and checks each GET /api/v1/jobs/{id}
answer: the requested id, the expected kind, a known status, a result
and a text or null error. An answer that is not the job is exposed as the
error and stops polling, so an unknown status no longer leaves the
Discover skeleton cards running with no error or Retry. useJob also
returns poll, which asks for the job at once and restarts the backoff;
the Discover page's "Couldn't check on new side quests" Retry uses it
instead of posting another batch.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/discover/api.ts

@@ -29,6 +29,9 @@ export type Choice = 'try' | 'pass'
 // running it, which GET /api/v1/jobs/{id} reports on.
 export type Batch = { batch_id: string; job_id: string }
 
+// BATCH_JOB_KIND is the kind of the job running a suggestion batch.
+export const BATCH_JOB_KIND = 'suggestion_batch'
+
 // BatchResult is the suggestion batch job's result, published as it goes.
 export type BatchResult = { batch_id: string; stage: string; suggestion_ids: string[]; dropped: number }