feat: ✨ Add Elasticsearch indexer configuration

Introduces the configuration for the issue indexer using Elasticsearch, enabling the ISSUE_INDEXER feature.

Sets the ISSUE_INDEXER_ENABLED flag to true and specifies the connection string sourced from a secret.

Prepares for future enhancements by including placeholders for repository indexing options.

template/stacks/core/forgejo/values.yaml

@@ -64,11 +64,23 @@ gitea:
         secretKeyRef:
           name: postgres-forgejo-cloud-credentials
           key: password
+    - name: FORGEJO__indexer__ISSUE_INDEXER_CONN_STR
+      valueFrom:
+        secretKeyRef:
+          name: elasticsearch-cloud-credentials
+          key: connection-string
 
   admin:
     existingSecret: gitea-credential
 
   config:
+    indexer:
+      ISSUE_INDEXER_ENABLED: true
+      ISSUE_INDEXER_TYPE: elasticsearch
+      # TODO next
+      REPO_INDEXER_ENABLED: false
+      # REPO_INDEXER_TYPE: meilisearch # not yet working
+
     storage:
       MINIO_ENDPOINT: obs.eu-de.otc.t-systems.com:443
       STORAGE_TYPE: minio