feat: make Django configs drive backups and scheduling

Treat SQL-backed Django models as the source of truth for pobsync
configuration, exporting runtime YAML only as a compatibility layer for
the existing engine. Add a database-driven scheduler command, Docker
scheduler services, schedule run-state fields, and tests for scheduler,
config export, and retention behavior.

src/pobsync_backend/models.py

@@ -102,6 +102,10 @@ class ScheduleConfig(TimestampedModel):
     prune = models.BooleanField(default=False)
     prune_max_delete = models.PositiveIntegerField(default=10)
     prune_protect_bases = models.BooleanField(default=False)
+    last_due_key = models.CharField(max_length=32, blank=True)
+    last_started_at = models.DateTimeField(null=True, blank=True)
+    last_finished_at = models.DateTimeField(null=True, blank=True)
+    last_status = models.CharField(max_length=16, blank=True)
 
     class Meta:
         ordering = ["host__host"]