Add MariaDB integration and repositories

Introduce DB support and repos: add modules/database.js (connection pool + runDDL) and SQL DDL files (data/ddl/*) for servers, users, providers, identities and credentials. Add repository layers: users, servers, providers, identities, credentials. Update server.js to run DDL at startup. Update .env.example with DB settings and bump dependencies in package.json/package-lock.json (add mariadb, bump dotenv/helmet/path-to-regexp/zod and dev tool upgrades). Error handling and logging included for DDL and repo operations.

data/ddl/01_users.sql

@@ -0,0 +1,11 @@
+CREATE TABLE IF NOT EXISTS `users` (
+    `id` UUID PRIMARY KEY DEFAULT UUID(),
+    `serverId` UUID NULL,
+    `remoteId` VARCHAR(255) NULL,
+    `displayName` VARCHAR(255) NOT NULL,
+    `avatarUrl` TEXT NULL,
+    `createdAt` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT `fk_users_serverId` FOREIGN KEY (`serverId`) REFERENCES `servers` (`id`) ON DELETE CASCADE,
+    CONSTRAINT `uq_users_server_remote` UNIQUE (`serverId`, `remoteId`)
+)