feat: Passwordless cross-device authentication

- Arkitektur: docs/auth/passwordless-architecture.md
- Backend: iom/quixzoom-auth-service/ (FastAPI + Redis)
- Webb: quixzoom-market-pages/se/login/ (QR-kod + polling)
- App: iom/quixzoom-app/src/features/auth/ (push + deep links)

Flöde: QR-kod → app-godkännande → webb-inloggad

landvex/node_modules/merge-stream/index.js

@@ -0,0 +1,41 @@
+'use strict';
+
+const { PassThrough } = require('stream');
+
+module.exports = function (/*streams...*/) {
+  var sources = []
+  var output  = new PassThrough({objectMode: true})
+
+  output.setMaxListeners(0)
+
+  output.add = add
+  output.isEmpty = isEmpty
+
+  output.on('unpipe', remove)
+
+  Array.prototype.slice.call(arguments).forEach(add)
+
+  return output
+
+  function add (source) {
+    if (Array.isArray(source)) {
+      source.forEach(add)
+      return this
+    }
+
+    sources.push(source);
+    source.once('end', remove.bind(null, source))
+    source.once('error', output.emit.bind(output, 'error'))
+    source.pipe(output, {end: false})
+    return this
+  }
+
+  function isEmpty () {
+    return sources.length == 0;
+  }
+
+  function remove (source) {
+    sources = sources.filter(function (it) { return it !== source })
+    if (!sources.length && output.readable) { output.end() }
+  }
+}