(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/migrations/0015_notificationtarget_notificationdelivery.py

@@ -0,0 +1,54 @@
+# Generated by Django 5.2.14 on 2026-05-28 19:11
+
+import django.db.models.deletion
+import pobsync_backend.models
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('pobsync_backend', '0014_host_bwlimit_override'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='NotificationTarget',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('updated_at', models.DateTimeField(auto_now=True)),
+                ('name', models.CharField(max_length=128, unique=True)),
+                ('enabled', models.BooleanField(default=True)),
+                ('channel', models.CharField(choices=[('email', 'Email'), ('webhook', 'Webhook')], max_length=16)),
+                ('statuses', models.JSONField(blank=True, default=pobsync_backend.models.default_notification_statuses)),
+                ('email_to', models.TextField(blank=True)),
+                ('webhook_url', models.URLField(blank=True, max_length=1024)),
+                ('webhook_headers', models.JSONField(blank=True, default=dict)),
+                ('notes', models.TextField(blank=True)),
+                ('last_status', models.CharField(blank=True, max_length=16)),
+                ('last_error', models.TextField(blank=True)),
+                ('last_sent_at', models.DateTimeField(blank=True, null=True)),
+            ],
+            options={
+                'ordering': ['name'],
+            },
+        ),
+        migrations.CreateModel(
+            name='NotificationDelivery',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('status', models.CharField(choices=[('sent', 'Sent'), ('failed', 'Failed'), ('skipped', 'Skipped')], max_length=16)),
+                ('error', models.TextField(blank=True)),
+                ('payload', models.JSONField(blank=True, default=dict)),
+                ('created_at', models.DateTimeField(auto_now_add=True)),
+                ('run', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='notification_deliveries', to='pobsync_backend.backuprun')),
+                ('target', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='deliveries', to='pobsync_backend.notificationtarget')),
+            ],
+            options={
+                'verbose_name_plural': 'notification deliveries',
+                'ordering': ['-created_at', 'target__name'],
+                'constraints': [models.UniqueConstraint(fields=('target', 'run'), name='unique_notification_delivery_per_target_run')],
+            },
+        ),
+    ]