(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/views.py

@@ -32,13 +32,24 @@ from .forms import (
     HostConfigForm,
     IncompleteCleanupForm,
     ManualBackupForm,
+    NotificationTargetForm,
     RetentionApplyForm,
     SshCredentialGenerateForm,
     ScheduleConfigForm,
     SshCredentialForm,
 )
 from .host_ops import ensure_host_directories
-from .models import BackupRun, GlobalConfig, HostConfig, PurgedSnapshot, ScheduleConfig, SnapshotRecord, SshCredential
+from .models import (
+    BackupRun,
+    GlobalConfig,
+    HostConfig,
+    NotificationDelivery,
+    NotificationTarget,
+    PurgedSnapshot,
+    ScheduleConfig,
+    SnapshotRecord,
+    SshCredential,
+)
 from .preflight import collect_backup_gate, effective_host_config_preview, run_remote_preflight
 from .retention import run_incomplete_cleanup, run_sql_retention_apply, run_sql_retention_plan
 from .self_check import collect_self_checks, summarize_self_checks
@@ -320,6 +331,65 @@ def logs(request):
     return render(request, "pobsync_backend/logs.html", context)
 
 
+@staff_member_required
+def notification_targets(request):
+    targets = NotificationTarget.objects.order_by("name")
+    deliveries = NotificationDelivery.objects.select_related("target", "run", "run__host").order_by("-created_at")[:12]
+    return render(
+        request,
+        "pobsync_backend/notification_targets.html",
+        {
+            "targets": targets,
+            "deliveries": deliveries,
+        },
+    )
+
+
+@staff_member_required
+def create_notification_target(request):
+    if request.method == "POST":
+        form = NotificationTargetForm(request.POST)
+        if form.is_valid():
+            target = form.save()
+            messages.success(request, f"Notification target {target.name} created.")
+            return redirect("notification_targets")
+    else:
+        form = NotificationTargetForm()
+    return render(
+        request,
+        "pobsync_backend/notification_target_form.html",
+        {
+            "form": form,
+            "target": None,
+            "title": "New notification target",
+            "submit_label": "Create target",
+        },
+    )
+
+
+@staff_member_required
+def edit_notification_target(request, target_id: int):
+    target = get_object_or_404(NotificationTarget, id=target_id)
+    if request.method == "POST":
+        form = NotificationTargetForm(request.POST, instance=target)
+        if form.is_valid():
+            target = form.save()
+            messages.success(request, f"Notification target {target.name} updated.")
+            return redirect("notification_targets")
+    else:
+        form = NotificationTargetForm(instance=target)
+    return render(
+        request,
+        "pobsync_backend/notification_target_form.html",
+        {
+            "form": form,
+            "target": target,
+            "title": f"Edit notification target: {target.name}",
+            "submit_label": "Save target",
+        },
+    )
+
+
 @staff_member_required
 def runs_list(request):
     status = request.GET.get("status", "").strip()