fix(cron): accept named months/weekdays in cron schedules

`parse_schedule()` detected cron expressions with a digit-only field pattern
(`^[\d\*\-,/]+$`), so any field using named months or weekdays — `MON`, `JAN`,
and common ranges/lists like `MON-FRI` or `MON,WED,FRI` — failed detection and
fell through to a confusing "Invalid schedule" error, even though croniter
supports them and they're standard cron.

Allow letters in the field pattern so these route to croniter for validation.
Truly-invalid expressions (`0 9 * * FUNDAY`, `99 9 * * MON`) are still rejected
there with a clear "Invalid cron expression" message; duration/interval/ISO
parsing is unchanged.

Adds tests for named weekdays/months (incl. ranges and lists) and that an
invalid named field is still rejected.

cron/jobs.py

@@ -960,9 +960,13 @@ def parse_schedule(schedule: str) -> Dict[str, Any]:
 
     # Check for cron expression (5 or 6 space-separated fields)
     # Cron fields: minute hour day month weekday [year]
+    # Allow letters so named months/weekdays (JAN-DEC, MON-SUN, incl. ranges
+    # and lists like MON-FRI or MON,WED,FRI) are routed to croniter, which
+    # supports them. The previous digit-only pattern silently rejected these
+    # valid expressions as "Invalid schedule".
     parts = schedule.split()
     if len(parts) >= 5 and all(
-        re.match(r'^[\d\*\-,/]+$', p) for p in parts[:5]
+        re.match(r'^[A-Za-z\d\*\-,/]+$', p) for p in parts[:5]
     ):
         if not _ensure_croniter():
             raise ValueError("Cron expressions require 'croniter' package. Install with: pip install croniter")

tests/cron/test_jobs.py

@@ -180,6 +180,25 @@ class TestParseSchedule:
         assert result["kind"] == "cron"
         assert result["expr"] == "0 9 * * *"
 
+    def test_cron_named_weekdays_and_months(self):
+        # Named months/weekdays (and ranges/lists) are valid cron and must
+        # route to croniter, not be rejected as "Invalid schedule".
+        pytest.importorskip("croniter")
+        for expr in (
+            "0 9 * * MON",
+            "*/15 9-17 * * MON-FRI",
+            "0 9 1 JAN *",
+            "0 9 * * MON,WED,FRI",
+        ):
+            result = parse_schedule(expr)
+            assert result["kind"] == "cron", expr
+            assert result["expr"] == expr
+
+    def test_invalid_named_cron_still_rejected(self):
+        pytest.importorskip("croniter")
+        with pytest.raises(ValueError):
+            parse_schedule("0 9 * * FUNDAY")
+
     def test_iso_timestamp(self):
         result = parse_schedule("2030-01-15T14:00:00")
         assert result["kind"] == "once"