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/02_providers.sql

@@ -0,0 +1,8 @@
+CREATE TABLE IF NOT EXISTS `providers` (
+    `id` UUID PRIMARY KEY DEFAULT UUID(),
+    `name` VARCHAR(64) NOT NULL UNIQUE,
+    `issuerUrl` TEXT NOT NULL,
+    `issuerLogo` TEXT,
+    `isEnabled` BOOLEAN NOT NULL DEFAULT TRUE,
+    `createdAt` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+)