fix(07): CR-02 correct cron expression for pollIntervalMin >= 60

Values >=60 in the cron minute field silently misbehave (*/60 fires once per
hour, */90 fires once per hour, etc.). Use the hours field for intervals >=60:
  <60 min  → */N * * * *
  >=60 min → 0 */H * * * (H = floor(N/60))
Also adds @Max(1440) to DkvConfigDto to bound the field at 24 h.

apps/api/src/dkv/dkv-scheduler.service.ts

@@ -101,8 +101,15 @@ export class DkvSchedulerService implements OnModuleInit {
       /* Job not yet registered — this is expected on first call */
     }
 
-    // Create new cron job with computed expression: every N minutes
-    const cronExpr = `*/${intervalMin} * * * *`;
+    // Create new cron job with computed expression.
+    // Standard cron minute field only accepts 0–59; for longer intervals use the hours field.
+    let cronExpr: string;
+    if (intervalMin < 60) {
+      cronExpr = `*/${intervalMin} * * * *`;         // e.g. */15 * * * *
+    } else {
+      const hours = Math.floor(intervalMin / 60);
+      cronExpr = `0 */${hours} * * *`;               // e.g. 0 */2 * * *
+    }
     const job = new CronJobClass(cronExpr, () => {
       this.dkvService.processInbox(tenant).catch((err) =>
         this.logger.error(