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.

repositories/identitiesRepo.js

@@ -0,0 +1,49 @@
+const { pool } = require("../modules/database")
+const { DefaultError } = require("../errors/errors")
+
+async function findByUserId(userId) {
+    try {
+        const sql = "SELECT* FROM identities WHERE userId = ?"
+        const rows = await pool.query(sql, [userId])
+        return rows || null
+    } catch (error) {
+        throw new DefaultError(500, "Internal Server Error", error)
+    }
+}
+
+async function findByProviderAndSubject(providerId, providerUserId) {
+    try {
+        const sql = "SELECT* FROM identities WHERE providerId = ? AND providerUserId = ?"
+        const rows = await pool.query(sql, [providerId, providerUserId])
+        return rows[0] || null
+    } catch (error) {
+        throw new DefaultError(500, "Internal Server Error", error)
+    }
+}
+
+async function create(userId, providerId, providerUserId) {
+    try {
+        const sql = "INSERT INTO identities (userId, providerId, providerUserId) VALUES (?, ?, ?)"
+        const rows = await pool.query(sql, [userId, providerId, providerUserId])
+        return rows[0] || null
+    } catch (error) {
+        throw new DefaultError(500, "Internal Server Error", error)
+    }
+}
+
+async function remove(id) {
+    try {
+        const sql = "DELETE FROM identities WHERE id = ?"
+        const rows = await pool.query(sql, [id])
+        return rows.affectedRows > 0
+    } catch (error) {
+        throw new DefaultError(500, "Internal Server Error", error)
+    }
+}
+
+module.exports = {
+    create,
+    remove,
+    findByUserId,
+    findByProviderAndSubject
+}