[ee] fix(indexer): tell admins when ingress routes search to wrong pod

When the IndexReader is absent on the pod handling a search request but
another pod is actively holding the indexer lock, the EE handler now
returns a tailored error pointing at the ingress/load-balancer
configuration instead of the generic "indexer not running" message.

The indexer status endpoint reads the DB lock so it reports "running"
from any pod, but search endpoints need the in-memory IndexReader that
only exists on the lock holder. In multi-replica deployments this looks
like the indexer is healthy but every search 404s.

Companion: windmill-labs/windmill-ee-private#TBD

Fixes WIN-1968.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/.sqlx/query-9b88e522ecbe9fa67ef83e79ec5eb5c9c87999a877fcb7f23be75d991bba6e49.json

@@ -0,0 +1,22 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "SELECT last_locked_at FROM concurrency_locks WHERE id = $1",
+  "describe": {
+    "columns": [
+      {
+        "ordinal": 0,
+        "name": "last_locked_at",
+        "type_info": "Timestamp"
+      }
+    ],
+    "parameters": {
+      "Left": [
+        "Text"
+      ]
+    },
+    "nullable": [
+      false
+    ]
+  },
+  "hash": "9b88e522ecbe9fa67ef83e79ec5eb5c9c87999a877fcb7f23be75d991bba6e49"
+}

backend/ee-repo-ref.txt

@@ -1 +1 @@
-daffe7bb81cfcaca666c61de1ee838a44d60ebc2
+7dd43d1850813071cc18ba49ba090583e7321f4b