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/04_credentials.sql

@@ -0,0 +1,7 @@
+CREATE TABLE IF NOT EXISTS `credentials` (
+    `userId` UUID PRIMARY KEY,
+    `passwordHash` TEXT NOT NULL,
+    `updatedAt` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+
+    CONSTRAINT `fk_credentials_userId` FOREIGN KEY (`userId`) REFERENCES `users` (`id`) ON DELETE CASCADE
+)