(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/forms.py

@@ -9,7 +9,7 @@ from tempfile import TemporaryDirectory
 from django import forms
 from django.conf import settings
 
-from .models import GlobalConfig, HostConfig, ScheduleConfig, SshCredential
+from .models import BackupRun, GlobalConfig, HostConfig, NotificationTarget, ScheduleConfig, SshCredential
 from .scheduler import parse_cron_expr
 
 
@@ -153,6 +153,62 @@ class ManualBackupForm(forms.Form):
     )
 
 
+class NotificationTargetForm(forms.ModelForm):
+    TERMINAL_STATUS_CHOICES = (
+        (BackupRun.Status.SUCCESS, BackupRun.Status.SUCCESS.label),
+        (BackupRun.Status.WARNING, BackupRun.Status.WARNING.label),
+        (BackupRun.Status.FAILED, BackupRun.Status.FAILED.label),
+        (BackupRun.Status.CANCELLED, BackupRun.Status.CANCELLED.label),
+    )
+
+    statuses = forms.MultipleChoiceField(
+        choices=TERMINAL_STATUS_CHOICES,
+        widget=forms.CheckboxSelectMultiple,
+        initial=[choice[0] for choice in TERMINAL_STATUS_CHOICES],
+        help_text="Send notifications for these terminal run statuses.",
+    )
+    email_to = forms.CharField(
+        widget=forms.Textarea,
+        required=False,
+        help_text="One recipient per line, or comma-separated.",
+    )
+    webhook_headers = forms.JSONField(
+        required=False,
+        widget=forms.Textarea(attrs={"rows": 4}),
+        help_text='Optional JSON object with extra headers, for example {"Authorization": "Bearer ..."}.',
+    )
+
+    class Meta:
+        model = NotificationTarget
+        fields = (
+            "name",
+            "enabled",
+            "channel",
+            "statuses",
+            "email_to",
+            "webhook_url",
+            "webhook_headers",
+            "notes",
+        )
+        widgets = {
+            "notes": forms.Textarea,
+        }
+
+    def clean(self):
+        cleaned_data = super().clean()
+        channel = cleaned_data.get("channel")
+        if channel == NotificationTarget.Channel.EMAIL and not cleaned_data.get("email_to", "").strip():
+            self.add_error("email_to", "Email targets need at least one recipient.")
+        if channel == NotificationTarget.Channel.WEBHOOK and not cleaned_data.get("webhook_url"):
+            self.add_error("webhook_url", "Webhook targets need a URL.")
+        return cleaned_data
+
+    def clean_email_to(self) -> str:
+        value = self.cleaned_data.get("email_to", "")
+        recipients = [line.strip() for line in value.replace(",", "\n").splitlines() if line.strip()]
+        return "\n".join(recipients)
+
+
 class SshCredentialForm(forms.ModelForm):
     private_key_file = forms.FileField(
         required=False,