(feature) Add Django retention apply flow

Expose retention apply from the host retention plan page so planned
snapshot deletions can be executed from the Django UI.

The form requires explicit host confirmation, carries through the
selected retention kind and base-protection setting, and uses max_delete
as a deletion guard. The view delegates to the SQL retention apply
service and reports predictable pobsync errors back through Django
messages instead of surfacing a server error.

Add view coverage for confirmed deletion, invalid confirmation, and
POST-only enforcement.

src/pobsync_backend/views.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import json
+from pathlib import Path
 
 from django.contrib import messages
 from django.contrib.admin.views.decorators import staff_member_required
@@ -9,12 +10,19 @@ from django.db.models import Count
 from django.shortcuts import get_object_or_404, redirect, render
 from django.views.decorators.http import require_POST
 
-from pobsync.errors import ConfigError
+from pobsync.errors import PobsyncError
 
 from .backup_runner import queue_backup_run
-from .forms import CreateHostConfigForm, GlobalConfigForm, HostConfigForm, ManualBackupForm, ScheduleConfigForm
+from .forms import (
+    CreateHostConfigForm,
+    GlobalConfigForm,
+    HostConfigForm,
+    ManualBackupForm,
+    RetentionApplyForm,
+    ScheduleConfigForm,
+)
 from .models import BackupRun, GlobalConfig, HostConfig, ScheduleConfig, SnapshotRecord
-from .retention import run_sql_retention_plan
+from .retention import run_sql_retention_apply, run_sql_retention_plan
 from .snapshot_discovery import discover_snapshots, inspect_snapshot_discovery
 
 
@@ -196,7 +204,7 @@ def host_retention_plan(request, host: str):
     protect_bases = request.GET.get("protect_bases") in {"1", "true", "on", "yes"}
     try:
         plan = run_sql_retention_plan(host=host_config.host, kind=kind, protect_bases=protect_bases)
-    except ConfigError as exc:
+    except PobsyncError as exc:
         messages.error(request, str(exc))
         return redirect("host_detail", host=host_config.host)
     context = {
@@ -204,10 +212,51 @@ def host_retention_plan(request, host: str):
         "kind": kind,
         "protect_bases": protect_bases,
         "plan": plan,
+        "apply_form": RetentionApplyForm(
+            host_name=host_config.host,
+            initial={
+                "kind": kind,
+                "protect_bases": protect_bases,
+                "max_delete": len(plan["delete"]),
+            },
+        ),
     }
     return render(request, "pobsync_backend/retention_plan.html", context)
 
 
+@staff_member_required
+@require_POST
+def apply_host_retention(request, host: str):
+    host_config = get_object_or_404(HostConfig, host=host)
+    form = RetentionApplyForm(request.POST, host_name=host_config.host)
+    if not form.is_valid():
+        messages.error(request, "Retention apply confirmation is invalid.")
+        return redirect("host_retention_plan", host=host_config.host)
+
+    kind = form.cleaned_data["kind"]
+    protect_bases = bool(form.cleaned_data["protect_bases"])
+    try:
+        result = run_sql_retention_apply(
+            prefix=Path(settings.POBSYNC_HOME),
+            host=host_config.host,
+            kind=kind,
+            protect_bases=protect_bases,
+            yes=True,
+            max_delete=form.cleaned_data["max_delete"],
+        )
+    except PobsyncError as exc:
+        messages.error(request, str(exc))
+    else:
+        messages.success(request, f"Retention deleted {len(result['deleted'])} snapshot(s) for {host_config.host}.")
+
+    target = redirect("host_retention_plan", host=host_config.host)
+    query = f"kind={kind}"
+    if protect_bases:
+        query += "&protect_bases=1"
+    target["Location"] = f"{target['Location']}?{query}"
+    return target
+
+
 @staff_member_required
 def edit_host_config(request, host: str):
     host_config = get_object_or_404(HostConfig, host=host)