Add version footer, task history UI, and note version retention policy

- App.vue: quiet version footer (fetches /api/version, 0.45 opacity)
- TaskEditorView: History button + HistoryPanel integration (mirrors NoteEditorView)
- note_versions.py: raise MAX_VERSIONS 20→50; add 5-min minimum interval gate
  to prevent autosave exhausting history slots

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

frontend/src/views/TaskEditorView.vue

@@ -22,6 +22,7 @@ import MilestoneSelector from "@/components/MilestoneSelector.vue";
 import TaskLogSection from "@/components/TaskLogSection.vue";
 import InlineAssistPanel from "@/components/InlineAssistPanel.vue";
 import ConfirmDialog from "@/components/ConfirmDialog.vue";
+import HistoryPanel from "@/components/HistoryPanel.vue";
 
 const route = useRoute();
 const router = useRouter();
@@ -321,6 +322,7 @@ async function save() {
 }
 
 const showDeleteConfirm = ref(false);
+const showHistory = ref(false);
 
 function remove() {
   if (taskId.value) {
@@ -386,6 +388,7 @@ useEditorGuards(dirty, save);
         <button class="btn-save" @click="save" :disabled="saving">
           {{ saving ? "Saving..." : "Save" }}
         </button>
+        <button v-if="isEditing" class="btn-history" @click="showHistory = true">History</button>
         <button v-if="isEditing" class="btn-delete" @click="remove">
           Delete
         </button>
@@ -654,6 +657,15 @@ useEditorGuards(dirty, save);
       @confirm="confirmDelete"
       @cancel="showDeleteConfirm = false"
     />
+
+    <!-- History panel -->
+    <HistoryPanel
+      v-if="showHistory && taskId"
+      :note-id="taskId"
+      :current-body="body"
+      @restore="(b: string, t: string[]) => { body = b; tags = t; markDirty(); }"
+      @close="showHistory = false"
+    />
   </main>
 </template>
 
@@ -843,6 +855,22 @@ useEditorGuards(dirty, save);
   font-family: inherit;
 }
 
+/* History button */
+.btn-history {
+  padding: 0.45rem 1rem;
+  background: none;
+  border: 1px solid var(--color-border);
+  border-radius: var(--radius-sm);
+  cursor: pointer;
+  font-size: 0.875rem;
+  color: var(--color-text-secondary);
+  font-family: inherit;
+}
+.btn-history:hover {
+  border-color: var(--color-primary);
+  color: var(--color-primary);
+}
+
 /* Tag suggest row inside sidebar */
 .tag-suggest-row {
   display: flex;