Fix race in first-time login user creation (#96)

syncUserFromAuthelia() did a plain check-then-insert with no
transaction, so two simultaneous first-visit requests for the same
brand-new user (e.g. two tabs opened right after SSO login) could
race: the second INSERT hits users.username's UNIQUE KEY, which
mysqli throws on (uncaught, PHP 8.1+ default report mode) rather than
returning false. Switched to INSERT ... ON DUPLICATE KEY UPDATE
followed by a re-fetch by username, so the losing request updates the
winner's row instead of throwing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MGDKHiU5RJdo3dqQUDow3X

models/UserModel.php

@@ -98,19 +98,30 @@ class UserModel
             $user['groups'] = $groups;
             $user['is_admin'] = $isAdmin;
         } else {
-            // Create new user
+            // Create new user. Uses INSERT ... ON DUPLICATE KEY UPDATE (rather than
+            // a plain INSERT) so two concurrent first-visit requests for the same
+            // brand-new username can't race: the losing request updates the row the
+            // winner just created instead of throwing an uncaught duplicate-key
+            // exception (users.username has a UNIQUE KEY, and mysqli throws on
+            // constraint violation under PHP 8.1+'s default report mode).
             $insertStmt = $this->conn->prepare(
-                "INSERT INTO users (username, display_name, email, `groups`, is_admin, last_login) VALUES (?, ?, ?, ?, ?, NOW())"
+                "INSERT INTO users (username, display_name, email, `groups`, is_admin, last_login)
+                 VALUES (?, ?, ?, ?, ?, NOW())
+                 ON DUPLICATE KEY UPDATE
+                     display_name = VALUES(display_name),
+                     email        = VALUES(email),
+                     `groups`     = VALUES(groups),
+                     is_admin     = VALUES(is_admin),
+                     last_login   = NOW()"
             );
             $insertStmt->bind_param("ssssi", $username, $displayName, $email, $groups, $isAdmin);
             $insertStmt->execute();
-
-            $userId = $this->conn->insert_id;
             $insertStmt->close();
 
-            // Get the newly created user
-            $stmt = $this->conn->prepare("SELECT * FROM users WHERE user_id = ?");
-            $stmt->bind_param("i", $userId);
+            // Re-fetch by username — works whether this request won the insert or
+            // lost the race and only updated the winner's row.
+            $stmt = $this->conn->prepare("SELECT * FROM users WHERE username = ?");
+            $stmt->bind_param("s", $username);
             $stmt->execute();
             $result = $stmt->get_result();
             $user = $result->fetch_assoc();