feat: cloud functions

.firebaserc

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+{
+  "projects": {
+    "default": "expo-starter-9daef"
+  }
+}

firebase.json

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+{
+  "functions": [
+    {
+      "source": "functions",
+      "codebase": "default",
+      "disallowLegacyRuntimeConfig": true,
+      "ignore": [
+        "node_modules",
+        ".git",
+        "firebase-debug.log",
+        "firebase-debug.*.log",
+        "*.local"
+      ]
+    }
+  ]
+}

functions/.eslintrc.js

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+module.exports = {
+  root: true,
+  env: {
+    es6: true,
+    node: true,
+  },
+  extends: [
+    "eslint:recommended",
+    "plugin:import/errors",
+    "plugin:import/warnings",
+    "plugin:import/typescript",
+    "google",
+    "plugin:@typescript-eslint/recommended",
+  ],
+  parser: "@typescript-eslint/parser",
+  parserOptions: {
+    project: ["tsconfig.json", "tsconfig.dev.json"],
+    sourceType: "module",
+  },
+  ignorePatterns: [
+    "lib/**", // Ignore built files.
+    "generated/**", // Ignore generated files.
+  ],
+  plugins: [
+    "@typescript-eslint",
+    "import",
+  ],
+  rules: {
+    "quotes": ["error", "double"],
+    "import/no-unresolved": "off",
+    "indent": ["error", 2],
+  },
+};

functions/.gitignore

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+# Compiled JavaScript files
+lib/**/*.js
+lib/**/*.js.map
+
+# TypeScript v1 declaration files
+typings/
+
+# Node.js dependency directory
+node_modules/
+*.local

functions/package.json

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+{
+  "name": "functions",
+  "scripts": {
+    "lint": "ESLINT_USE_FLAT_CONFIG=false eslint --ext .js,.ts src",
+    "build": "tsc",
+    "build:watch": "tsc --watch",
+    "serve": "npm run build && firebase emulators:start --only functions",
+    "shell": "npm run build && firebase functions:shell",
+    "start": "npm run shell",
+    "deploy": "firebase deploy --only functions",
+    "logs": "firebase functions:log"
+  },
+  "engines": {
+    "node": "24"
+  },
+  "main": "lib/index.js",
+  "dependencies": {
+    "firebase-admin": "^13.6.0",
+    "firebase-functions": "^7.0.0"
+  },
+  "devDependencies": {
+    "@typescript-eslint/eslint-plugin": "^5.12.0",
+    "@typescript-eslint/parser": "^5.12.0",
+    "eslint": "^8.9.0",
+    "eslint-config-google": "^0.14.0",
+    "eslint-plugin-import": "^2.25.4",
+    "firebase-functions-test": "^3.4.1",
+    "typescript": "^6.0.0"
+  },
+  "private": true
+}

functions/src/index.ts

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+import {setGlobalOptions} from "firebase-functions/v2";
+
+setGlobalOptions({maxInstances: 10});
+
+export * from "./users";

functions/src/users.ts

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+import {getApps, initializeApp} from "firebase-admin/app";
+import {getAuth} from "firebase-admin/auth";
+import * as logger from "firebase-functions/logger";
+import {onDocumentDeleted} from "firebase-functions/v2/firestore";
+
+const adminApp = getApps().length > 0 ? getApps()[0] : initializeApp();
+
+export const deleteUserAuthOnUserDocumentDeleted = onDocumentDeleted(
+  "users/{uid}",
+  async (event) => {
+    const uid = event.params.uid;
+
+    if (!uid) {
+      logger.error("Missing uid in users/{uid} delete trigger.");
+      return;
+    }
+
+    try {
+      await getAuth(adminApp).deleteUser(uid);
+      logger.info(
+        "Deleted auth user after Firestore user deletion.",
+        {uid}
+      );
+    } catch (error) {
+      logger.error(
+        "Failed to delete auth user after Firestore user deletion.",
+        {
+          uid,
+          error,
+        }
+      );
+      throw error;
+    }
+  }
+);

functions/tsconfig.dev.json

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+{
+  "include": [
+    ".eslintrc.js"
+  ]
+}

functions/tsconfig.json

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+{
+  "compilerOptions": {
+    "module": "NodeNext",
+    "esModuleInterop": true,
+    "moduleResolution": "nodenext",
+    "noImplicitReturns": true,
+    "noUnusedLocals": true,
+    "outDir": "lib",
+    "rootDir": "src",
+    "sourceMap": true,
+    "strict": true,
+    "target": "es2017"
+  },
+  "compileOnSave": true,
+  "include": [
+    "src"
+  ]
+}