fix: admin-only suppression enforcement, links.html broken date parsing

Security: add require_admin decorator; apply to POST/DELETE /api/suppressions
and /suppressions page. Previously any user in allowed_groups could create or
delete suppressions even though the nav restricts the UI to admins.

Bug: links.html "Updated:" timestamp and stale-warning both produced
Invalid Date because the raw "YYYY-MM-DD HH:MM:SS UTC" string was appended
with 'Z' instead of being normalised through _toIso(). Fix both call sites to
use _toIso(), and remove the now-redundant local _toIso redefinition.

Style: use `with open(sentinel, 'w'): pass` consistently (was open().close()
at avatar JPEG validation path).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/links.html

@@ -36,7 +36,6 @@
 {% block scripts %}
 <script>
 const escHtml = s => lt.escHtml(s);
-const _toIso  = s => s ? s.replace(' UTC', 'Z').replace(' ', 'T') : s;
 
 // ── Formatting helpers ────────────────────────────────────────────
 function fmtRate(bytesPerSec) {
@@ -527,7 +526,7 @@ function expandAll() {
 // ── Stale data warning ────────────────────────────────────────────
 function checkLinksStale(updatedStr) {
   if (!updatedStr) return;
-  const age = (Date.now() - new Date(updatedStr + (updatedStr.includes('Z') ? '' : 'Z'))) / 1000;
+  const age = (Date.now() - new Date(_toIso(updatedStr))) / 1000;
   let banner = document.getElementById('links-stale-banner');
   if (age > 120) {
     if (!banner) {
@@ -556,7 +555,7 @@ async function loadLinks() {
     }
     const updEl = document.getElementById('links-updated');
     if (updEl && data.updated) {
-      updEl.textContent = 'Updated: ' + new Date(data.updated + (data.updated.includes('Z') ? '' : 'Z')).toLocaleTimeString();
+      updEl.textContent = 'Updated: ' + new Date(_toIso(data.updated)).toLocaleTimeString();
     }
     renderLinks(data);
     checkLinksStale(data.updated);