admin/index.php: add server-side pagination (25/page)

- Add Database::getThesesListCount(array $filters) — runs the same WHERE
  clauses as getThesesList() but with COUNT(DISTINCT t.id); used to compute
  total pages without loading all rows.
- Extend Database::getThesesList() with $limit/$offset parameters; when
  $limit > 0 appends LIMIT/OFFSET and re-binds positional params individually
  to avoid the PDO mixed-style restriction.
- Fix getThesesList() SELECT: add LEFT JOIN access_types + at.name as
  access_type — the column was referenced in the template but never fetched.
- Wire admin/index.php: read ?page=, compute $totalPages/$offset, pass
  $perPage=25 + $offset to getThesesList(); include pagination.php partial
  below the table with filter-preserving $baseParams.
- Add result-count line (<p class="admin-list-meta">) showing "X–Y sur Z TFE"
  when multiple pages exist.
- Add .admin-body .pagination-wrap / .pagination-btn / .pagination-info styles
  to admin.css (scoped to .admin-body to avoid colliding with public pages).

TODO.md

@@ -11,6 +11,8 @@ Pending tasks have been split into topic files under [`todo/`](todo/README.md):
 
 ## Recently completed (this session)
 
+- [x] `admin/index.php` — server-side pagination (25/page); `Database::getThesesListCount()` added; `getThesesList()` extended with `$limit`/`$offset`; `access_type` JOIN added to query (was missing); result-count meta line added; `.pagination-wrap` + `.pagination-btn` + `.pagination-info` styles added to `admin.css`
+
 - [x] `checkbox-list.php` — replaced `<div class="admin-checkbox-list">` with `<fieldset class="admin-checkbox-group"><legend class="sr-only">…</legend><ul>` (WCAG 1.3.1 fix)
 - [x] `admin.css` — replaced `.admin-checkbox-list` with `.admin-body fieldset.admin-checkbox-group > ul` semantic selectors; added `span.admin-row-label` as visible label column counterpart
 - [x] `login.php` — wrapped content in `<main id="main-content">` landmark