perf: replace fat-view student index query with lean getPublishedAuthors()

The répertoire page was loading the full v_theses_public view
(15 JOINs + 8 GROUP_CONCAT temp B-trees) via getAllPublishedTheses()
just to build the student name → thesis-id map on the index page.
Only two columns (id, authors) were ever consumed by the template.

Add Database::getPublishedAuthors(): array
- Queries thesis_authors JOIN authors directly on the theses base table
- Filters on theses.is_published = 1 using the existing index
- Returns only id + GROUP_CONCAT(authors) — no view expansion
- Results verified identical to the old getAllPublishedTheses() output

Update search.php to call getPublishedAuthors() instead.
Mark getAllPublishedTheses() @deprecated in Database.php.

All tests pass.

public/search.php

@@ -46,8 +46,8 @@ try {
     $orientations = $db->getAllOrientations();
     $apPrograms   = $db->getAllAPPrograms();
     $keywords     = $db->getUsedTags();
-    // Fetch all published theses for the student index (no artificial cap)
-    $students = $db->getAllPublishedTheses();
+    // Fetch id+authors only — lean query bypassing the fat v_theses_public view
+    $students = $db->getPublishedAuthors();
 } catch (InvalidArgumentException $e) {
     $validationError = $e->getMessage();
     $results = []; $totalItems = 0; $totalPages = 0;