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.

TODO.md

@@ -51,6 +51,9 @@
 - [x] #fix-desync-deploy-code-exit-23 Fix desync: deploy-code exit-23 'Operation not permitted' — drop -p/-t (destination owned by www-data:xamxam, setgid; SSH user can't chmod), use -rlDz --size-only --context deploy
 - [x] #deploy-permissions-sudo-prompt-can-t deploy-permissions sudo prompt can't accept input: ssh -t drops pty when local stdin isn't a TTY. Add scoped NOPASSWD sudo drop-in (deploy/xamxam-fix-permissions.sudoers) + deploy-sudoers recipe and wire into deploy --context deploy
 - [x] #fix-raw-markdown-leaking Fix raw markdown leaking into TOC labels on licence/charte/about pages
+- [x] #rewrite-cc2r-checkbox-label Rewrite CC2r checkbox label to 'J'adhère au Collective Commitment to Reuse (CC2r)' (italic, both Libre and Interne branches)
+- [x] #fix-admin-contenus-page [!high] Fix admin contenus page slowdown: langues/mots-clés fragments ship 2.4MB HTML (737 tag rows + 217 lang rows, ~1960 inline SVG icons + per-row CSRF forms). Implement htmx infinite-scroll 'load more' (paged fragments via limit/offset) for both tables.
+- [x] #reduce-contenus-initial-page [!high] Reduce contenus initial page size from 100 to 25 rows per table (payload ~160KB total vs ~534KB)
 
 ## Deferred / Blocked
 - [ ] #just-setup-backs-a [!medium] just setup backs a stale setup-dev.sh (clones php-live-reload, legacy admin/data/ dirs) — needs rewrite or removal