Add MONGO_URI and admin Basic Auth config

Add mongoUri and admin.{user,password} config keys to config/env/all.js
for Task 2 admin transaction management feature. Follows existing env var
pattern used for payment provider credentials. Default mongoUri points to
Docker service; admin credentials read from ADMIN_USER/ADMIN_PASSWORD env.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.env.example

@@ -42,3 +42,8 @@ ALEPAY_CHECKSUM_KEY=
 # Appota
 APPOTA_API_KEY=
 APPOTA_SECRET_KEY=
+
+# Admin transaction management
+MONGO_URI=
+ADMIN_USER=
+ADMIN_PASSWORD=

config/env/all.js

@@ -108,5 +108,10 @@ module.exports = {
     cancel_url: "https://pg.cbotvietnam.com/appota/cancel",
     ipn_url: "https://pg.cbotvietnam.com/appota/ipn",
   },
+  mongoUri: process.env.MONGO_URI || "mongodb://mongo:27017/haiyen_admin",
+  admin: {
+    user: process.env.ADMIN_USER,
+    password: process.env.ADMIN_PASSWORD,
+  },
   encrypt_key: process.env.ENCRYPT_KEY,
 };