Load the backend's env files from its own directory

The root `bun test` also runs the backend's tests, from the repository root, where
dotenv-flow found no backend/.env: they passed only while the root .env still carried
stale SEMAPPS_* lines, and failed on a clean one (HOME_URL undefined).

backend/config/config.js

@@ -1,4 +1,6 @@
-require('dotenv-flow').config();
+// The backend's own env files, wherever the process starts: the root `bun test` runs the
+// backend's tests too, from the repository root.
+require('dotenv-flow').config({ path: require('path').join(__dirname, '..') });
 
 module.exports = {
   APP_NAME: process.env.SEMAPPS_APP_NAME,