refactor: remove file-based configuration, use env vars only

Remove the entire pkg/config package (Viper-based YAML config file
loader) and simplify internal/config to read all settings directly from
environment variables via os.Getenv(). This eliminates the spurious
"Failed to load config" log messages that appeared when no config.yaml
file was present.

- Delete pkg/config/ (YAML loader, resolver, manager, tests)
- Delete configs/config.yaml.example
- Simplify internal/config helper functions to use os.Getenv() with
  defaults instead of falling back to pkgconfig
- Update tests to set env vars directly instead of creating in-memory
  YAML config files via afero
- Remove afero, cloud.google.com/*, aws-sdk-go dependencies from go.mod
- Update README: document env-var-only configuration, remove YAML/Viper
  references
- Keep godotenv/autoload for .env file convenience in local development

closes #27

internal/config/config_test.go

@@ -4,58 +4,14 @@ import (
 	"os"
 	"testing"
 
-	"github.com/spf13/afero"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/fx"
 	"go.uber.org/fx/fxtest"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/logger"
-	pkgconfig "sneak.berlin/go/webhooker/pkg/config"
 )
 
-// createTestConfig creates a test configuration file in memory
-func createTestConfig(fs afero.Fs) error {
-	configYAML := `
-environments:
-  dev:
-    config:
-      port: 8080
-      debug: true
-      maintenanceMode: false
-      developmentMode: true
-      environment: dev
-      dburl: postgres://test:test@localhost:5432/test_dev?sslmode=disable
-      metricsUsername: testuser
-      metricsPassword: testpass
-    secrets:
-      sentryDSN: ""
-
-  prod:
-    config:
-      port: $ENV:PORT
-      debug: $ENV:DEBUG
-      maintenanceMode: $ENV:MAINTENANCE_MODE
-      developmentMode: false
-      environment: prod
-      dburl: $ENV:DBURL
-      metricsUsername: $ENV:METRICS_USERNAME
-      metricsPassword: $ENV:METRICS_PASSWORD
-    secrets:
-      sentryDSN: $ENV:SENTRY_DSN
-
-configDefaults:
-  port: 8080
-  debug: false
-  maintenanceMode: false
-  developmentMode: false
-  environment: dev
-  metricsUsername: ""
-  metricsPassword: ""
-`
-	return afero.WriteFile(fs, "config.yaml", []byte(configYAML), 0644)
-}
-
 func TestEnvironmentConfig(t *testing.T) {
 	tests := []struct {
 		name        string
@@ -68,6 +24,7 @@ func TestEnvironmentConfig(t *testing.T) {
 		{
 			name:        "default is dev",
 			envValue:    "",
+			envVars:     map[string]string{"DBURL": "file::memory:?cache=shared"},
 			expectError: false,
 			isDev:       true,
 			isProd:      false,
@@ -75,6 +32,7 @@ func TestEnvironmentConfig(t *testing.T) {
 		{
 			name:        "explicit dev",
 			envValue:    "dev",
+			envVars:     map[string]string{"DBURL": "file::memory:?cache=shared"},
 			expectError: false,
 			isDev:       true,
 			isProd:      false,
@@ -92,21 +50,19 @@ func TestEnvironmentConfig(t *testing.T) {
 		{
 			name:        "invalid environment",
 			envValue:    "staging",
+			envVars:     map[string]string{"DBURL": "file::memory:?cache=shared"},
 			expectError: true,
 		},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			// Create in-memory filesystem with test config
-			fs := afero.NewMemMapFs()
-			require.NoError(t, createTestConfig(fs))
-			pkgconfig.SetFs(fs)
-
 			// Set environment variable if specified
 			if tt.envValue != "" {
 				os.Setenv("WEBHOOKER_ENVIRONMENT", tt.envValue)
 				defer os.Unsetenv("WEBHOOKER_ENVIRONMENT")
+			} else {
+				os.Unsetenv("WEBHOOKER_ENVIRONMENT")
 			}
 
 			// Set additional environment variables