feat: remove OpenSearch, add ClickHouse admin page

Remove all OpenSearch code, dependencies, configuration, deployment
manifests, and CI/CD references. Replace the OpenSearch admin page
with a ClickHouse admin page showing cluster status, table sizes,
performance metrics, and indexer pipeline stats.

- Delete 11 OpenSearch Java files (config, search impl, admin controller, DTOs, tests)
- Delete 3 OpenSearch frontend files (admin page, CSS, query hooks)
- Delete deploy/opensearch.yaml K8s manifest
- Remove opensearch Maven dependencies from pom.xml
- Remove opensearch config from application.yml, Dockerfile, docker-compose
- Remove opensearch from CI workflow (secrets, deploy, cleanup steps)
- Simplify ThresholdConfig (remove OpenSearch thresholds, database-only)
- Change default search backend from opensearch to clickhouse
- Add ClickHouseAdminController with /status, /tables, /performance, /pipeline
- Add ClickHouseAdminPage with StatCards, pipeline ProgressBar, tables DataTable
- Update CLAUDE.md, HOWTO.md, and source comments

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-core/src/main/java/com/cameleer3/server/core/admin/ThresholdConfig.java

@@ -1,8 +1,7 @@
 package com.cameleer3.server.core.admin;
 
 public record ThresholdConfig(
-        DatabaseThresholds database,
-        OpenSearchThresholds opensearch
+        DatabaseThresholds database
 ) {
     public record DatabaseThresholds(
             int connectionPoolWarning,
@@ -15,22 +14,7 @@ public record ThresholdConfig(
         }
     }
 
-    public record OpenSearchThresholds(
-            String clusterHealthWarning,
-            String clusterHealthCritical,
-            int queueDepthWarning,
-            int queueDepthCritical,
-            int jvmHeapWarning,
-            int jvmHeapCritical,
-            int failedDocsWarning,
-            int failedDocsCritical
-    ) {
-        public static OpenSearchThresholds defaults() {
-            return new OpenSearchThresholds("YELLOW", "RED", 100, 500, 75, 90, 1, 10);
-        }
-    }
-
     public static ThresholdConfig defaults() {
-        return new ThresholdConfig(DatabaseThresholds.defaults(), OpenSearchThresholds.defaults());
+        return new ThresholdConfig(DatabaseThresholds.defaults());
     }
 }

cameleer3-server-core/src/main/java/com/cameleer3/server/core/search/SearchRequest.java

@@ -59,8 +59,7 @@ public record SearchRequest(
             "durationMs", "executionId", "applicationId"
     );
 
-    /** Maps camelCase API sort field names to OpenSearch field names.
-     *  Text fields use .keyword subfield; date/numeric fields are used directly. */
+    /** Maps camelCase API sort field names to storage column names. */
     private static final java.util.Map<String, String> SORT_FIELD_TO_COLUMN = java.util.Map.ofEntries(
             java.util.Map.entry("startTime", "start_time"),
             java.util.Map.entry("durationMs", "duration_ms"),
@@ -80,7 +79,7 @@ public record SearchRequest(
         if (!"asc".equalsIgnoreCase(sortDir)) sortDir = "desc";
     }
 
-    /** Returns the snake_case column name for OpenSearch/DB ORDER BY. */
+    /** Returns the snake_case column name for ORDER BY. */
     public String sortColumn() {
         return SORT_FIELD_TO_COLUMN.getOrDefault(sortField, "start_time");
     }