Server-side sorting for execution search results

Sorting now applies to the entire result set via ClickHouse ORDER BY
instead of only sorting the current page client-side. Default sort
order is timestamp descending. Supported sort columns: startTime,
status, agentId, routeId, correlationId, durationMs.

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

ui/src/api/openapi.json

@@ -255,6 +255,22 @@
               "format": "int32",
               "default": 50
             }
+          },
+          {
+            "name": "sortField",
+            "in": "query",
+            "required": false,
+            "schema": {
+              "type": "string"
+            }
+          },
+          {
+            "name": "sortDir",
+            "in": "query",
+            "required": false,
+            "schema": {
+              "type": "string"
+            }
           }
         ],
         "responses": {
@@ -1500,6 +1516,12 @@
           "limit": {
             "type": "integer",
             "format": "int32"
+          },
+          "sortField": {
+            "type": "string"
+          },
+          "sortDir": {
+            "type": "string"
           }
         }
       },