chore: Refactor test setup for Gin framework

- Removed redundant `gin.SetMode(gin.TestMode)` calls from individual test files.
- Introduced a centralized `TestMain` function in `testmain_test.go` to set the Gin mode for all tests.
- Ensured consistent test environment setup across various handler test files.

backend/internal/api/handlers/security_priority_test.go

@@ -19,7 +19,6 @@ import (
 // 2. SecurityConfig DB (middle)
 // 3. Static config (lowest)
 func TestSecurityHandler_Priority_SettingsOverSecurityConfig(t *testing.T) {
-	gin.SetMode(gin.TestMode)
 
 	tests := []struct {
 		name              string
@@ -112,7 +111,6 @@ func TestSecurityHandler_Priority_SettingsOverSecurityConfig(t *testing.T) {
 
 // TestSecurityHandler_Priority_AllModules verifies priority system works for all security modules
 func TestSecurityHandler_Priority_AllModules(t *testing.T) {
-	gin.SetMode(gin.TestMode)
 	db := setupTestDB(t)
 	require.NoError(t, db.AutoMigrate(&models.Setting{}, &models.SecurityConfig{}))