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.

src/Database.php

@@ -408,6 +408,10 @@ class Database {
     /**
      * Return ALL published theses (no cap) — for internal use (student index).
      * Not exposed to user-controlled input.
+     *
+     * @deprecated Use getPublishedAuthors() for the répertoire student index —
+     *             it avoids the expensive v_theses_public view and only fetches
+     *             the two columns actually needed (id, authors).
      */
     public function getAllPublishedTheses(): array {
         $stmt = $this->pdo->query(
@@ -416,6 +420,29 @@ class Database {
         return $stmt->fetchAll();
     }
 
+    /**
+     * Lean query for the répertoire student-name index.
+     *
+     * Avoids the fat v_theses_public view (15 JOINs + 8 GROUP_CONCAT B-trees).
+     * Queries thesis_authors → authors directly, letting the index on
+     * theses(is_published) filter the base table before any JOIN.
+     *
+     * Returns rows of [id => int, authors => "Name1,Name2"].
+     */
+    public function getPublishedAuthors(): array {
+        $stmt = $this->pdo->query(
+            "SELECT t.id,
+                    GROUP_CONCAT(a.name ORDER BY ta.author_order ASC) AS authors
+             FROM theses t
+             JOIN thesis_authors ta ON ta.thesis_id = t.id
+             JOIN authors a         ON a.id          = ta.author_id
+             WHERE t.is_published = 1
+             GROUP BY t.id
+             ORDER BY MIN(a.name) ASC"
+        );
+        return $stmt->fetchAll();
+    }
+
     public function getAvailableYears() {
         $sql = "SELECT DISTINCT year FROM theses WHERE is_published = 1 ORDER BY year DESC";
         $stmt = $this->pdo->query($sql);