(feature) Add run completion notifications

Add email and webhook notification targets with delivery tracking, and send
notifications when backup runs reach a terminal status.

Expose notification target management in the Django UI and keep delivery
failures recorded without failing the backup worker.

src/pobsync_backend/models.py

@@ -135,6 +135,63 @@ class BackupRun(models.Model):
         return f"{self.host} {self.run_type} {self.status}"
 
 
+def default_notification_statuses() -> list[str]:
+    return [
+        BackupRun.Status.SUCCESS,
+        BackupRun.Status.WARNING,
+        BackupRun.Status.FAILED,
+        BackupRun.Status.CANCELLED,
+    ]
+
+
+class NotificationTarget(TimestampedModel):
+    class Channel(models.TextChoices):
+        EMAIL = "email", "Email"
+        WEBHOOK = "webhook", "Webhook"
+
+    name = models.CharField(max_length=128, unique=True)
+    enabled = models.BooleanField(default=True)
+    channel = models.CharField(max_length=16, choices=Channel.choices)
+    statuses = models.JSONField(default=default_notification_statuses, blank=True)
+    email_to = models.TextField(blank=True)
+    webhook_url = models.URLField(max_length=1024, blank=True)
+    webhook_headers = models.JSONField(default=dict, blank=True)
+    notes = models.TextField(blank=True)
+    last_status = models.CharField(max_length=16, blank=True)
+    last_error = models.TextField(blank=True)
+    last_sent_at = models.DateTimeField(null=True, blank=True)
+
+    class Meta:
+        ordering = ["name"]
+
+    def __str__(self) -> str:
+        return self.name
+
+
+class NotificationDelivery(models.Model):
+    class Status(models.TextChoices):
+        SENT = "sent", "Sent"
+        FAILED = "failed", "Failed"
+        SKIPPED = "skipped", "Skipped"
+
+    target = models.ForeignKey(NotificationTarget, on_delete=models.CASCADE, related_name="deliveries")
+    run = models.ForeignKey(BackupRun, on_delete=models.CASCADE, related_name="notification_deliveries")
+    status = models.CharField(max_length=16, choices=Status.choices)
+    error = models.TextField(blank=True)
+    payload = models.JSONField(default=dict, blank=True)
+    created_at = models.DateTimeField(auto_now_add=True)
+
+    class Meta:
+        constraints = [
+            models.UniqueConstraint(fields=["target", "run"], name="unique_notification_delivery_per_target_run"),
+        ]
+        ordering = ["-created_at", "target__name"]
+        verbose_name_plural = "notification deliveries"
+
+    def __str__(self) -> str:
+        return f"{self.target} run {self.run_id} {self.status}"
+
+
 class SnapshotRecord(models.Model):
     class Kind(models.TextChoices):
         SCHEDULED = "scheduled", "Scheduled"