ignore *.db files, fix thesis identifier to use max seq instead of count, untrack .db files

app/src/Database.php

@@ -1524,11 +1524,22 @@ class Database {
      * number. Must be called inside the same transaction that performs the INSERT so that
      * concurrent requests cannot produce duplicate identifiers.
      */
+    /**
+     * Generate a unique identifier like "2025-003" for a new thesis.
+     *
+     * Uses the actual maximum sequence number for the given year (from
+     * existing identifiers) rather than the row count, so deletes don't
+     * cause identifier collisions.
+     *
+     * Must be called inside an open transaction.
+     */
     public function generateThesisIdentifier(int $year): string {
-        $stmt = $this->pdo->prepare("SELECT COUNT(*) FROM theses WHERE year = ?");
-        $stmt->execute([$year]);
-        $count = (int)$stmt->fetchColumn() + 1;
-        return sprintf("%d-%03d", $year, $count);
+        $stmt = $this->pdo->prepare(
+            "SELECT COALESCE(MAX(CAST(SUBSTR(identifier, 6) AS INTEGER)), 0) FROM theses WHERE identifier LIKE ?"
+        );
+        $stmt->execute([$year . '-%']);
+        $maxSeq = (int)$stmt->fetchColumn();
+        return sprintf("%d-%03d", $year, $maxSeq + 1);
     }
 
     /**