Add Jest to support backend unit tests

backend/jest.config.ts

@@ -0,0 +1,17 @@
+import type { Config } from "@jest/types"
+
+const config: Config.InitialOptions = {
+  preset: "ts-jest",
+  testEnvironment: "node",
+  verbose: true,
+  automock: false,
+  collectCoverage: true,
+  collectCoverageFrom: ["./src/**/**.ts"],
+  coverageProvider: "v8",
+  coverageThreshold: {
+    global: {
+      lines: 90
+    }
+  }
+}
+export default config;