(feature) Add read-only retention plan view

Add a staff-only retention plan page for each host using the SQL-backed
retention service. Link it from the host detail page and show policy settings,
keep reasons, and snapshots that would be deleted for scheduled, manual, or all
snapshot kinds.

Keep the flow non-destructive for now, validate query parameters, and cover the
view with tests for rendering, base protection, and invalid kind handling.

src/pobsync_backend/templates/pobsync_backend/base.html

@@ -80,7 +80,7 @@
     .stack { display: grid; gap: 4px; }
     .two-col { display: grid; gap: 18px; grid-template-columns: minmax(0, 1fr) minmax(0, 1fr); }
     .actions { display: flex; flex-wrap: wrap; gap: 10px; margin: 0 0 18px; }
-    button {
+    button, .button-link {
       appearance: none;
       background: #17202a;
       border: 1px solid #17202a;
@@ -91,7 +91,7 @@
       font-weight: 650;
       padding: 8px 12px;
     }
-    button:hover { background: #2a394a; }
+    button:hover, .button-link:hover { background: #2a394a; text-decoration: none; }
     .messages { display: grid; gap: 8px; margin-bottom: 18px; }
     .message {
       background: var(--panel);

src/pobsync_backend/templates/pobsync_backend/host_detail.html

@@ -10,6 +10,7 @@
       {% csrf_token %}
       <button type="submit">Discover snapshots</button>
     </form>
+    <a class="button-link" href="{% url 'host_retention_plan' host.host %}">Plan retention</a>
   </section>
 
   <section class="grid" aria-label="Host summary">

src/pobsync_backend/templates/pobsync_backend/retention_plan.html

@@ -0,0 +1,83 @@
+{% extends "pobsync_backend/base.html" %}
+
+{% block title %}Retention plan | {{ host.host }}{% endblock %}
+
+{% block content %}
+  <h1>Retention Plan: {{ host.host }}</h1>
+
+  <section class="actions" aria-label="Retention filters">
+    <a class="button-link" href="{% url 'host_detail' host.host %}">Back to host</a>
+    <a class="button-link" href="{% url 'host_retention_plan' host.host %}?kind=scheduled">Scheduled</a>
+    <a class="button-link" href="{% url 'host_retention_plan' host.host %}?kind=manual">Manual</a>
+    <a class="button-link" href="{% url 'host_retention_plan' host.host %}?kind=all">All</a>
+    <a class="button-link" href="{% url 'host_retention_plan' host.host %}?kind={{ kind }}&protect_bases=1">Protect bases</a>
+  </section>
+
+  <section class="grid" aria-label="Retention plan summary">
+    <div class="metric"><div class="label">Source</div><div class="value">{{ plan.source }}</div></div>
+    <div class="metric"><div class="label">Kind</div><div class="value">{{ plan.kind }}</div></div>
+    <div class="metric"><div class="label">Keep</div><div class="value">{{ plan.keep|length }}</div></div>
+    <div class="metric"><div class="label">Would Delete</div><div class="value">{{ plan.delete|length }}</div></div>
+  </section>
+
+  <section class="panel">
+    <h2>Policy</h2>
+    <div class="stack">
+      <div><strong>Daily:</strong> {{ plan.retention.daily }}</div>
+      <div><strong>Weekly:</strong> {{ plan.retention.weekly }}</div>
+      <div><strong>Monthly:</strong> {{ plan.retention.monthly }}</div>
+      <div><strong>Yearly:</strong> {{ plan.retention.yearly }}</div>
+      <div><strong>Protect bases:</strong> {{ protect_bases|yesno:"yes,no" }}</div>
+    </div>
+  </section>
+
+  <section class="panel">
+    <h2>Would Delete</h2>
+    <table>
+      <thead>
+        <tr>
+          <th>Kind</th>
+          <th>Dirname</th>
+          <th>Started</th>
+          <th>Status</th>
+          <th>Path</th>
+        </tr>
+      </thead>
+      <tbody>
+        {% for snapshot in plan.delete %}
+          <tr>
+            <td>{{ snapshot.kind }}</td>
+            <td>{{ snapshot.dirname }}</td>
+            <td>{{ snapshot.dt }}</td>
+            <td>{{ snapshot.status|default:"" }}</td>
+            <td class="muted">{{ snapshot.path }}</td>
+          </tr>
+        {% empty %}
+          <tr><td colspan="5" class="muted">Retention would not delete snapshots for this selection.</td></tr>
+        {% endfor %}
+      </tbody>
+    </table>
+  </section>
+
+  <section class="panel">
+    <h2>Keep Reasons</h2>
+    <table>
+      <thead>
+        <tr>
+          <th>Dirname</th>
+          <th>Reasons</th>
+        </tr>
+      </thead>
+      <tbody>
+        {% for dirname, reasons in plan.reasons.items %}
+          <tr>
+            <td>{{ dirname }}</td>
+            <td>{{ reasons|join:", " }}</td>
+          </tr>
+        {% empty %}
+          <tr><td colspan="2" class="muted">No snapshots matched this retention selection.</td></tr>
+        {% endfor %}
+      </tbody>
+    </table>
+  </section>
+{% endblock %}

src/pobsync_backend/tests/test_views.py

@@ -100,6 +100,58 @@ class ViewTests(TestCase):
 
         self.assertEqual(response.status_code, 405)
 
+    def test_retention_plan_renders_keep_and_delete_sets(self) -> None:
+        self.client.force_login(self.staff_user)
+        host = HostConfig.objects.create(
+            host="web-01",
+            address="web-01.example.test",
+            retention_daily=0,
+            retention_weekly=0,
+            retention_monthly=0,
+            retention_yearly=0,
+        )
+        old_snapshot = self._snapshot(host, "20260518-021500Z__OLDSNAP")
+        new_snapshot = self._snapshot(host, "20260519-021500Z__NEWSNAP")
+
+        response = self.client.get(reverse("host_retention_plan", args=[host.host]))
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "Retention Plan: web-01")
+        self.assertContains(response, old_snapshot.dirname)
+        self.assertContains(response, new_snapshot.dirname)
+        self.assertContains(response, "newest")
+        self.assertContains(response, "Would Delete")
+
+    def test_retention_plan_can_enable_base_protection(self) -> None:
+        self.client.force_login(self.staff_user)
+        host = HostConfig.objects.create(
+            host="web-01",
+            address="web-01.example.test",
+            retention_daily=0,
+            retention_weekly=0,
+            retention_monthly=0,
+            retention_yearly=0,
+        )
+        base = self._snapshot(host, "20260518-021500Z__BASESNAP")
+        child = self._snapshot(host, "20260519-021500Z__CHILDSNP")
+        child.base = base
+        child.save(update_fields=["base"])
+
+        response = self.client.get(reverse("host_retention_plan", args=[host.host]), {"protect_bases": "1"})
+
+        self.assertEqual(response.status_code, 200)
+        self.assertContains(response, "Protect bases:</strong> yes")
+        self.assertContains(response, f"base-of:{child.dirname}")
+
+    def test_retention_plan_rejects_invalid_kind(self) -> None:
+        self.client.force_login(self.staff_user)
+        host = HostConfig.objects.create(host="web-01", address="web-01.example.test")
+
+        response = self.client.get(reverse("host_retention_plan", args=[host.host]), {"kind": "weird"}, follow=True)
+
+        self.assertRedirects(response, reverse("host_detail", args=[host.host]))
+        self.assertContains(response, "Retention kind must be scheduled, manual, or all.")
+
     def _snapshot(self, host: HostConfig, dirname: str) -> SnapshotRecord:
         started_at = datetime.strptime(dirname.split("__", 1)[0], "%Y%m%d-%H%M%SZ").replace(tzinfo=timezone.utc)
         return SnapshotRecord.objects.create(

src/pobsync_backend/views.py

@@ -6,7 +6,10 @@ 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 .models import BackupRun, HostConfig, ScheduleConfig, SnapshotRecord
+from .retention import run_sql_retention_plan
 from .snapshot_discovery import discover_snapshots
 
 
@@ -70,6 +73,28 @@ def discover_host_snapshots(request, host: str):
     return redirect("host_detail", host=host_config.host)
 
 
+@staff_member_required
+def host_retention_plan(request, host: str):
+    host_config = get_object_or_404(HostConfig, host=host)
+    kind = request.GET.get("kind", "scheduled")
+    if kind not in {"scheduled", "manual", "all"}:
+        messages.error(request, "Retention kind must be scheduled, manual, or all.")
+        return redirect("host_detail", host=host_config.host)
+    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:
+        messages.error(request, str(exc))
+        return redirect("host_detail", host=host_config.host)
+    context = {
+        "host": host_config,
+        "kind": kind,
+        "protect_bases": protect_bases,
+        "plan": plan,
+    }
+    return render(request, "pobsync_backend/retention_plan.html", context)
+
+
 def _schedule_for_host(host_config: HostConfig) -> ScheduleConfig | None:
     try:
         return host_config.schedule