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/03_identities.sql

@@ -0,0 +1,11 @@
+CREATE TABLE IF NOT EXISTS `identities` (
+    `id` UUID PRIMARY KEY DEFAULT UUID(),
+    `userId` UUID NOT NULL,
+    `providerId` UUID NOT NULL,
+    `providerUserId` VARCHAR(255) NOT NULL,
+    `createdAt` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+
+    CONSTRAINT `fk_identities_userId` FOREIGN KEY (`userId`) REFERENCES `users` (`id`) ON DELETE CASCADE,
+    CONSTRAINT `fk_identities_providerId` FOREIGN KEY (`providerId`) REFERENCES `providers` (`id`) ON DELETE CASCADE,
+    CONSTRAINT `uq_identities_provider_subject` UNIQUE (`providerId`, `providerUserId`)
+)