Add Discord OAuth2 account linking and login support

Introduces Discord OAuth2 integration for account association and login, including new routes for linking, unlinking, and authenticating via Discord. Adds supporting services, repositories, and schema validation for the OAuth2 flow. Refactors database schema and queries for consistency, and updates dependencies to include required OAuth2 libraries.

services/userService.js

@@ -29,6 +29,10 @@ async function getPlayerProperty(uuid, key) {
     return await userRepository.getPlayerProperty(key, uuid)
 }
 
+async function getPlayerPropertyByValue(key, value) {
+    return await userRepository.getPlayerPropertyByValue(key, value)
+}
+
 async function addPlayerProperty(uuid, key, value) {
     return await userRepository.addPropertyToPlayer(key, value, uuid)
 }
@@ -561,6 +565,7 @@ module.exports = {
     getPlayerCertificate,
     savePlayerCertificate,
     clearAllPlayerActions,
+    getPlayerPropertyByValue,
     getPlayerNameChangeStatus,
     getPlayerUsernamesHistory,
     deleteExpiredCertificates,