Pick and show schedules as plain time instead of cron

Schedules were entered and displayed as six field cron expressions. A
picker now offers a frequency, a weekday or day of the month and a time,
and turns that into the expression the backend stores; tables and the
dashboard show wording like "Every day at 03:00". An expression the
picker cannot express stays editable as cron.

This also fixes a broken preset: the backend's cron dialect counts Sunday
as 1 and rejects 0, so the "every Sunday" choice in the application
backup form produced an invalid expression. The picker emits weekday
names, and a test pins the dialect down.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/crates/application/src/tests/scheduler_tests.rs

@@ -31,3 +31,50 @@ fn due_when_a_fire_time_lies_between_last_run_and_now() {
     assert!(!is_due(hourly, None, now + Duration::minutes(10), 300));
     assert!(!is_due("bad", None, now, 300));
 }
+
+/// The UI offers simple choices ("every Sunday at 3:00") and turns them into these
+/// expressions, so the dialect they have to match is pinned down here.
+#[test]
+fn documents_the_cron_dialect_the_ui_generates() {
+    let fires_at = |expr: &str, after: chrono::DateTime<Utc>| next_fire(expr, after).unwrap();
+    let monday = Utc.with_ymd_and_hms(2026, 9, 7, 0, 0, 0).unwrap(); // a Monday
+
+    // every day at 3:00
+    assert_eq!(
+        fires_at("0 0 3 * * *", monday),
+        Utc.with_ymd_and_hms(2026, 9, 7, 3, 0, 0).unwrap()
+    );
+    // every six hours, on the hour
+    assert_eq!(
+        fires_at("0 0 */6 * * *", monday),
+        Utc.with_ymd_and_hms(2026, 9, 7, 6, 0, 0).unwrap()
+    );
+    // every hour at half past
+    assert_eq!(
+        fires_at("0 30 * * * *", monday),
+        Utc.with_ymd_and_hms(2026, 9, 7, 0, 30, 0).unwrap()
+    );
+    // on the first day of the month at 3:30
+    assert_eq!(
+        fires_at("0 30 3 1 * *", monday),
+        Utc.with_ymd_and_hms(2026, 10, 1, 3, 30, 0).unwrap()
+    );
+
+    // Weekdays are 1-7 with Sunday as 1, and names work; a zero is not a valid day.
+    let sunday = Utc.with_ymd_and_hms(2026, 9, 13, 3, 0, 0).unwrap();
+    assert_eq!(fires_at("0 0 3 * * SUN", monday), sunday);
+    assert_eq!(
+        fires_at("0 0 3 * * 1", monday),
+        sunday,
+        "1 is Sunday, not Monday"
+    );
+    assert_eq!(
+        fires_at("0 0 3 * * MON", monday),
+        Utc.with_ymd_and_hms(2026, 9, 7, 3, 0, 0).unwrap()
+    );
+    assert!(
+        validate_cron("0 0 3 * * 0").is_err(),
+        "a zero weekday is rejected"
+    );
+    assert_eq!(next_fire("0 0 3 * * 0", monday), None);
+}