(feature) Show next scheduled run and backup run type in the UI

Add a scheduler helper that calculates the next due time for a cron-style
schedule expression and surface that value on the dashboard and host detail
pages.

Show the latest run type in host summaries and backup trend tables so
manual and scheduled backups are distinguishable in the Django UI.

Keep the calculation derived from existing ScheduleConfig data without
adding a migration.

src/pobsync_backend/scheduler.py

@@ -1,7 +1,7 @@
 from __future__ import annotations
 
 from dataclasses import dataclass
-from datetime import datetime
+from datetime import datetime, timedelta
 
 
 @dataclass(frozen=True)
@@ -36,6 +36,17 @@ def is_due(expr: str, moment: datetime) -> bool:
     )
 
 
+def next_due_after(expr: str, moment: datetime, *, max_days: int = 366) -> datetime | None:
+    parse_cron_expr(expr)
+    candidate = moment.replace(second=0, microsecond=0) + timedelta(minutes=1)
+    deadline = candidate + timedelta(days=max_days)
+    while candidate <= deadline:
+        if is_due(expr, candidate):
+            return candidate
+        candidate += timedelta(minutes=1)
+    return None
+
+
 def _field_matches(field: str, value: int, min_value: int, max_value: int, sunday_alias: bool = False) -> bool:
     for part in field.split(","):
         if _part_matches(part.strip(), value, min_value, max_value, sunday_alias=sunday_alias):