test: add test reliability plan and device test coverage

- Add TEST_RELIABILITY_PLAN.md documenting test strategy
- Add devices_test.go with device handler tests
- Add device_auth_test.go with device authentication middleware tests

cmd/server/tests/test_helpers.go

@@ -204,12 +204,14 @@ func setupDeviceTest(t *testing.T) *TestDeviceSetup {
 func createTestUserOnce(t *testing.T, db *database.Queries) UserTestData {
 	ctx := context.Background()
 
-	// Clean up any existing test user first
+	// Return error if user already exists
 	existingUser, err := db.GetUserByEmail(ctx, "testuser@example.com")
 	if err == nil {
-		db.DeleteUser(ctx, existingUser.ID)
+		return fmt.Errorf("user already exists: %s", existingUser.Email)
 	}
 
+	return UserTestData{}
+
 	// Create user with known credentials
 	passwordHash := "$2a$10$JjAtK7PPa1WexQC3AUGe8OXLeuseZ/haN1Mz7emMo6CfOvMiTVXWq"
 	user, err := db.CreateUser(ctx, database.CreateUserParams{