Add comparison stats: failure rate %, vs-yesterday change, today total

Stats endpoint now returns current + previous period (24h shift) values
plus today's total count. UI shows:
- Total Matches: "of 12.3K today"
- Avg Duration: arrow + % vs yesterday
- Failure Rate: percentage of errors vs total, arrow + % vs yesterday
- P99 Latency: arrow + % vs yesterday
- In-Flight: unchanged (running executions)

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

ui/src/api/openapi.json

@@ -1049,6 +1049,10 @@
       "ExecutionStats": {
         "type": "object",
         "properties": {
+          "totalCount": {
+            "type": "integer",
+            "format": "int64"
+          },
           "failedCount": {
             "type": "integer",
             "format": "int64"
@@ -1064,6 +1068,26 @@
           "activeCount": {
             "type": "integer",
             "format": "int64"
+          },
+          "totalToday": {
+            "type": "integer",
+            "format": "int64"
+          },
+          "prevTotalCount": {
+            "type": "integer",
+            "format": "int64"
+          },
+          "prevFailedCount": {
+            "type": "integer",
+            "format": "int64"
+          },
+          "prevAvgDurationMs": {
+            "type": "integer",
+            "format": "int64"
+          },
+          "prevP99LatencyMs": {
+            "type": "integer",
+            "format": "int64"
           }
         }
       },