Add cancel button for active indexing jobs (CS-11135)

[lukemelia]

• Relies on CS-11141: Allow X-Grafana-Device-Id in realm-server CORS preflight #4835

Summary

• Adds a per-row Cancel action on the Active Indexing panel (indexing.json) for currently-running indexing jobs.
• The button POSTs to the existing _grafana-complete-job endpoint with the row's reservation_id; the handler (handle-remove-job.ts) already resolves it via findJobIdForReservationId and calls forceCancelJobById. No server-side change required.
• Mirrors the existing Cancel running reservation pattern in job-queue.json's Running Jobs panel, so all the auth + confirmation plumbing matches what we ship today.

Linear: CS-11135

Test plan

• Apply dashboards locally (packages/observability/scripts/apply.sh against a local Grafana) and confirm the Active Indexing panel renders a red Cancel cell instead of a hidden column.
• Start an indexing job locally, click Cancel, confirm the prompt, and verify the row disappears and the job is marked rejected (result.status = 418) with finished_at set.
• Verify no regression on the Running Jobs panel in job-queue.json.

🤖 Generated with Claude Code

[github-actions]

Grafana preview

Preview deployed for 1 dashboard in the staging Grafana.
Cross-dashboard drill-throughs still point at the canonical staging dashboards.

• Folder: Preview – PR #4836: Boxel Status

Dashboards:

• [PR #4836] Indexing

Preview is torn down automatically when this PR is closed or merged.

(Run: https://github.com/cardstack/boxel/actions/runs/25877458051)

[github-actions]

Observability diff (vs staging)

Show diff

diff --git a/tmp/remote-canon.1hFnGi/dashboards/boxel-status/indexing.json b/tmp/committed-canon.OhDATU/dashboards/boxel-status/indexing.json
index 23fa4b3..a4a9e79 100644
--- a/tmp/remote-canon.1hFnGi/dashboards/boxel-status/indexing.json
+++ b/tmp/committed-canon.OhDATU/dashboards/boxel-status/indexing.json
@@ -69,6 +69,10 @@
           "uid": "cef5v5sl9k7i8f"
         },
         "description": "System-wide operator action: queue a full reindex across every realm. The button disables itself while a `full-reindex` orchestration job is already pending or running. Per-realm reindex moved to the Realms dashboard. Click POSTs with `Authorization: Bearer ${grafana_secret}` (substituted from SSM at apply time, CS-10929).",
+        "fieldConfig": {
+          "defaults": {},
+          "overrides": []
+        },
         "gridPos": {
           "h": 8,
           "w": 24,
@@ -714,8 +718,57 @@
               },
               "properties": [
                 {
-                  "id": "custom.hidden",
-                  "value": true
+                  "id": "actions",
+                  "value": [
+                    {
+                      "confirmation": "Cancel running indexing job (reservation ${__value.raw})? The worker will stop processing it.",
+                      "fetch": {
+                        "body": "",
+                        "headers": [
+                          [
+                            "Authorization",
+                            "Bearer ${grafana_secret}"
+                          ]
+                        ],
+                        "method": "POST",
+                        "queryParams": [
+                          [
+                            "reservation_id",
+                            "${__value.raw}"
+                          ]
+                        ],
+                        "url": "${realm_server}_grafana-complete-job"
+                      },
+                      "oneClick": false,
+                      "title": "Cancel reservation ${__value.raw}",
+                      "type": "fetch"
+                    }
+                  ]
+                },
+                {
+                  "id": "mappings",
+                  "value": [
+                    {
+                      "options": {
+                        "from": 0,
+                        "result": {
+                          "color": "red",
+                          "index": 0,
+                          "text": "Cancel"
+                        },
+                        "to": 9999999999999
+                      },
+                      "type": "range"
+                    }
+                  ]
+                },
+                {
+                  "id": "displayName",
+                  "value": "Action"
+                },
+                {
+                  "id": "custom.filterable",
+                  "value": false
                 }
               ]
             },

(Run: https://github.com/cardstack/boxel/actions/runs/25877458009)