fix(admin): serialize poll response as JSON string via ObjectMapper

Response.ok(Map.of(...)).build() with class-level @Produces(TEXT_HTML)
caused Quarkus to call Map.toString() instead of the Jackson writer,
producing {key=value} output that r.json() can't parse. Explicitly
serialize with ObjectMapper and set Content-Type to application/json.

Co-Authored-By: Mira Rehfeld <noreply@anthropic.com>

apix-admin/src/main/java/org/botstandards/apix/admin/resource/DashboardResource.java

@@ -56,10 +56,11 @@ public class DashboardResource {
             SandboxDashboardResponse d = registry.sandboxDetail(adminKey, uuid);
             long total = d.usage() == null ? 0L :
                     d.usage().values().stream().mapToLong(Long::longValue).sum();
-            return Response.ok(Map.of(
+            String json = mapper.writeValueAsString(Map.of(
                     "totalRequests", total,
                     "ratePerMinute", d.ratePerMinute()
-            )).build();
+            ));
+            return Response.ok(json).type(MediaType.APPLICATION_JSON).build();
         } catch (Exception e) {
             LOG.errorf(e, "Poll failed for sandbox %s", uuid);
             return Response.serverError().build();