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>

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

@@ -1,11 +1,17 @@
 package com.cameleer3.server.core.search;
 
 /**
- * Aggregate execution statistics within a time window.
- *
- * @param failedCount   number of failed executions
- * @param avgDurationMs average duration in milliseconds
- * @param p99LatencyMs  99th percentile duration in milliseconds
- * @param activeCount   number of currently running executions
+ * Aggregate execution statistics within a time window, with comparison to the
+ * equivalent previous period (shifted back 24 h) and a "today" total.
  */
-public record ExecutionStats(long failedCount, long avgDurationMs, long p99LatencyMs, long activeCount) {}
+public record ExecutionStats(
+        long totalCount,
+        long failedCount,
+        long avgDurationMs,
+        long p99LatencyMs,
+        long activeCount,
+        long totalToday,
+        long prevTotalCount,
+        long prevFailedCount,
+        long prevAvgDurationMs,
+        long prevP99LatencyMs) {}