refactor: flatten monorepo structure to backend/ frontend/ devops/

Rename subdirectories for a cleaner single-repo layout:
- website-monitoring-backend/  → backend/
- website-monitoring-frontend/ → frontend/
- website-monitoring-devops/   → devops/

Update all references in package.json scripts, CI workflows,
docker-compose, pre-commit hooks, and documentation.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/eslint.config.mjs

@@ -0,0 +1,23 @@
+import eslint from "@typescript-eslint/eslint-plugin";
+import parser from "@typescript-eslint/parser";
+
+export default [
+  {
+    files: ["src/**/*.ts"],
+    languageOptions: {
+      parser,
+      parserOptions: {
+        ecmaVersion: 2022,
+        sourceType: "module",
+      },
+    },
+    plugins: {
+      "@typescript-eslint": eslint,
+    },
+    rules: {
+      "@typescript-eslint/no-unused-vars": ["warn", { argsIgnorePattern: "^_" }],
+      "@typescript-eslint/no-explicit-any": "warn",
+      "no-console": "off",
+    },
+  },
+];