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/database/webhook_db_manager_test.go

@@ -7,32 +7,20 @@ import (
 	"testing"
 
 	"github.com/google/uuid"
-	"github.com/spf13/afero"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 	"go.uber.org/fx/fxtest"
 	"sneak.berlin/go/webhooker/internal/config"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/logger"
-	pkgconfig "sneak.berlin/go/webhooker/pkg/config"
 )
 
 func setupTestWebhookDBManager(t *testing.T) (*WebhookDBManager, *fxtest.Lifecycle) {
 	t.Helper()
 
-	fs := afero.NewMemMapFs()
-	testConfigYAML := `
-environments:
-  dev:
-    config:
-      port: 8080
-      debug: false
-      dburl: "file::memory:?cache=shared"
-configDefaults:
-  port: 8080
-`
-	require.NoError(t, afero.WriteFile(fs, "config.yaml", []byte(testConfigYAML), 0644))
-	pkgconfig.SetFs(fs)
+	// Set DBURL env var for config loading
+	os.Setenv("DBURL", "file::memory:?cache=shared")
+	t.Cleanup(func() { os.Unsetenv("DBURL") })
 
 	lc := fxtest.NewLifecycle(t)
 
@@ -52,7 +40,6 @@ configDefaults:
 		DBURL:   "file::memory:?cache=shared",
 		DataDir: dataDir,
 	}
-	_ = cfg
 
 	mgr, err := NewWebhookDBManager(lc, WebhookDBManagerParams{
 		Config: cfg,