🐛 Declare the transit content type on management calls

Every poller turn died on `claim-job` with `missing-key` for `job-id`
and `scheduled-at`: the JSON calls never declared their content type,
so the backend never parsed the body as transit and validated an empty
map. The header goes on the transit calls only; the multipart settle
keeps the content type its own FormData settles.

AI-assisted-by: glm-5.3-flash

exporter/src/app/consumer/api.cljs

@@ -69,11 +69,14 @@
                                    :response body)))))))))
 
 (defn- request
-  "POST `params` to a management method as transit+json."
+  "POST `params` to a management method as transit+json. The content
+  type travels on every call: without it the backend never parses the
+  body as transit and validates an empty map."
   [method params]
   (->> (fetch (method-uri method)
               #js {:method     "POST"
-                   :headers    (shared-key)
+                   :headers    (doto (shared-key)
+                                 (unchecked-set "Content-Type" "application/transit+json"))
                    :body       (t/encode-str params)
                    :dispatcher (dispatcher)})
        (p/mcat parse-response)))

exporter/test/exporter_tests/api_test.cljs

@@ -95,6 +95,19 @@
         restore!
         done))))
 
+(t/deftest json-calls-declare-the-transit-content-type
+  ;; without it the backend never parses the body as transit and
+  ;; validates an empty map (every key a `missing-key`)
+  (t/async done
+    (let [restore! (install-fetch!)]
+      (with-restore-and-done
+        (p/fmap (fn [_]
+                  (t/is (= "application/transit+json"
+                           (fetch-header "Content-Type"))))
+                (api/claim-job (uuid/next) "2026-10-07T00:00:00Z"))
+        restore!
+        done))))
+
 (t/deftest report-job-progress-posts-the-beat
   (t/async done
     (let [job-id   (uuid/next)