feat(cms): optimistic locking against stale-tab overwrites

A Content Editor tab left open across a deploy (or a save from another tab)
held the pre-deploy content; one Save would silently overwrite the newer
file. The editor page now embeds a SHA-256 fingerprint of the file content
at load time, /save requires it back in X-Content-Hash and compares against
the current file: mismatch (or a missing header) answers 409 with an
explanatory message and writes nothing. The client offers a reload on 409.

Integration test covers: matching hash saves, stale hash rejected with the
file untouched, missing hash rejected, retry with the fresh hash succeeds.

Closes MITHOME-61

scripts/cms-editor-client.js

@@ -233,10 +233,16 @@ async function save() {
   const data = collect();
   const res = await fetch('/save?file=' + FILE, {
     method: 'POST',
-    headers: { 'Content-Type': 'application/json', 'X-CSRF-Token': CSRF_TOKEN },
+    headers: { 'Content-Type': 'application/json', 'X-CSRF-Token': CSRF_TOKEN, 'X-Content-Hash': CONTENT_HASH },
     body: JSON.stringify(data, null, 2)
   });
   if (res.status === 401) { location.href = '/login'; return; }
+  if (res.status === 409) {
+    if (confirm('A tartalom megváltozott, mióta ez a lap megnyílt (pl. deploy vagy másik fül mentett).\n\nOK = lap frissítése az új tartalommal (a szerkesztésed elvész)\nMégse = maradsz ezen a lapon, a mentés nem történt meg.')) {
+      location.reload();
+    }
+    return;
+  }
   const json = await res.json();
   const status = document.getElementById('saveStatus');
   if (json.ok) {