feat(admin): rename series route and add tags review page

Rename /admin/series-management to /admin/series so it follows the
/admin/<section> convention; the breadcrumb's auto-derived parent link
is now a real route (was a dead /admin/series link).

Add an /admin/tags page to review pending TagSuggestions — list,
approve (creates the Tag), reject — backed by new admin endpoints and a
tagSuggestionReviewSchema. Resolves the dead /admin/tags alert link.

app/layouts/admin.vue

@@ -44,7 +44,7 @@
           </li>
           <li>
             <NuxtLink
-              to="/admin/series-management"
+              to="/admin/series"
               :class="{ active: route.path.includes('/admin/series') }"
             >
               Series
@@ -66,6 +66,14 @@
               Board Channels
             </NuxtLink>
           </li>
+          <li>
+            <NuxtLink
+              to="/admin/tags"
+              :class="{ active: route.path.startsWith('/admin/tags') }"
+            >
+              Tags
+            </NuxtLink>
+          </li>
           <li>
             <NuxtLink
               to="/admin/site-content"
@@ -153,7 +161,7 @@
               </li>
               <li>
                 <NuxtLink
-                  to="/admin/series-management"
+                  to="/admin/series"
                   :class="{ active: route.path.includes('/admin/series') }"
                   @click="isMobileMenuOpen = false"
                 >
@@ -178,6 +186,15 @@
                   Board Channels
                 </NuxtLink>
               </li>
+              <li>
+                <NuxtLink
+                  to="/admin/tags"
+                  :class="{ active: route.path.startsWith('/admin/tags') }"
+                  @click="isMobileMenuOpen = false"
+                >
+                  Tags
+                </NuxtLink>
+              </li>
               <li>
                 <NuxtLink
                   to="/admin/site-content"