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/schema.d.ts

@@ -217,10 +217,16 @@ export interface ProcessorNode {
 export type ProcessorSnapshot = Record<string, string>;
 
 export interface ExecutionStats {
+  totalCount: number;
   failedCount: number;
   avgDurationMs: number;
   p99LatencyMs: number;
   activeCount: number;
+  totalToday: number;
+  prevTotalCount: number;
+  prevFailedCount: number;
+  prevAvgDurationMs: number;
+  prevP99LatencyMs: number;
 }
 
 export interface StatsTimeseries {