docs: align Bruno tests with Go integration test credentials

- Update Login User.bru to use testuser@example.com
- Update Register User.bru to use testuser@example.com
- Update Register Admin User.bru to use maxdevices@example.com
- Standardize password to Test@Pass123! across all tests
- Add TEST_DATA.md documenting shared test credentials
- Update bruno.json with documentation reference
- Add comments linking to TEST_DATA.md for cross-reference

This alignment makes it easier to verify test failures between
Bruno API tests and Go integration tests using identical credentials.

bruno/user/auth/Login User.bru

@@ -12,11 +12,14 @@ post {
 
 body:json {
   {
-    "login": "test@example.com",
-    "password": "Password123!"
+    "login": "testuser@example.com",
+    "password": "Test@Pass123!"
   }
 }
 
+// Test credentials align with Go integration tests
+// See TEST_DATA.md for shared test data documentation
+
 script:post-response {
   function onResponse(res) {
     let data = res.getBody();

bruno/user/auth/Register User.bru

@@ -12,14 +12,17 @@ post {
 
 body:json {
   {
-    "email": "test@example.com",
+    "email": "testuser@example.com",
     "username": "testuser",
-    "password": "Password123!",
+    "password": "Test@Pass123!",
     "first_name": "Test",
     "last_name": "User"
   }
 }
 
+// Test user aligns with Go integration tests
+// See TEST_DATA.md for shared test data documentation
+
 script:post-response {
   function onResponse(res) {
     let data = res.getBody();