perf(admin): infinite-scroll the contenus langues/mots-clés tables

The contenus page loaded both lookup tables in full via HTMX on page load:
760 tag rows + 216 language rows, ~2.4MB of HTML including ~1960 inline
SVG icons and per-row CSRF forms. The DB queries were already fast (~6ms);
the cost was pure client-side payload and DOM.

- Add paged lookups: getTagsPage/getLanguagesPage + countTagsWithCount/
  countLanguagesWithCount, and optional limit/offset on the existing
  unpaged variants (backward compatible).
- Fragments serve 25 rows per request. A sentinel <tr> with
  hx-trigger="intersect once root:#<wrap>" appends the next page when it
  scrolls into the table's own scroll container (htmx 'revealed' only
  checks the window viewport, not nested scrollers).
- Search forms still swap the whole wrapper and reset to offset 0.
- Style the load-more row; add PagedLanguagesTagsTest coverage.

Initial DOM for the page drops from ~2.4MB to ~160KB; scrolling reaches the
full totals (736 tags, 216 languages) and stops cleanly.

app/public/assets/css/admin.css

@@ -255,6 +255,22 @@
   text-align: center;
 }
 
+/* "Load more" sentinel row for infinite-scroll admin tables (contenus).
+   Sits at the bottom of the <tbody>; htmx replaces it with the next page
+   of rows when it scrolls into the table's viewport. */
+.admin-load-more__cell {
+  color: var(--text-tertiary);
+  padding: var(--space-s) var(--space-2xs);
+  text-align: center;
+  font-size: var(--step--1);
+}
+
+/* Fade the sentinel out once its request is in flight so it does not
+   visually "stick" while the next page is fetched. */
+.admin-load-more.htmx-request .admin-load-more__cell {
+  opacity: 0.4;
+}
+
 /* Identifier column in the thesis table */
 .admin-table-id {
   color: var(--text-secondary);