feat: show-closed toggle, closed badge, reopen button, bulk purge

GET /web/cases now honours ?show_closed=1: the toggle link flips the
listing between open-only (default) and open+closed (muted styling,
grey "geschlossen" badge, countdown "wird in N Tagen entfernt" when
auto_delete_days > 0). Each closed row shows a Lucide rotate-ccw
reopen button in the same slot where the close button sits on open
cases; selection checkboxes are suppressed so bulk-close cannot touch
closed cases.

GET /web/cases/:id honours ?show_closed=1 too: without the query a
closed case still 404s (keeps the default listing clean), with it the
detail page renders the reopen form in the header. The case_page
template swaps the icon based on is_closed.

The purge-closed bulk button from the earlier commit finally gets a
UI: it appears below the listing when show_closed=1 is active AND at
least one closed case is visible, with a JS confirm() + the
server-side confirm=yes guard. Two integration tests (listing +
detail) cover the happy path.

server/templates/case_page.html

@@ -261,6 +261,37 @@
                 >{% else %}<span class="status-badge open">offen</span>{% endif
                 %}
             </span>
+            {% if is_closed %}
+            <form
+                class="delete-form"
+                method="post"
+                action="/web/cases/{{ case_id }}/reopen"
+            >
+                <button
+                    type="submit"
+                    class="delete-btn"
+                    title="Fall wiedereröffnen"
+                    aria-label="Fall wiedereröffnen"
+                >
+                    <svg
+                        xmlns="http://www.w3.org/2000/svg"
+                        viewBox="0 0 24 24"
+                        width="18"
+                        height="18"
+                        fill="none"
+                        stroke="currentColor"
+                        stroke-width="2"
+                        stroke-linecap="round"
+                        stroke-linejoin="round"
+                    >
+                        <path
+                            d="M3 12a9 9 0 1 0 9-9 9.75 9.75 0 0 0-6.74 2.74L3 8"
+                        ></path>
+                        <path d="M3 3v5h5"></path>
+                    </svg>
+                </button>
+            </form>
+            {% else %}
             <form
                 class="delete-form"
                 method="post"
@@ -293,6 +324,7 @@
                     </svg>
                 </button>
             </form>
+            {% endif %}
         </h1>
         {% match recorded_at_iso %} {% when Some with (iso) %}
         <p class="case-time">