chore: run go fix for Go 1.26 modernization

106 files: interface{}→any, strings.Split→SplitSeq, CutPrefix/Cut, strings.Builder, slices.Contains, remove redundant // +build directives, gofmt import ordering and indentation.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

backend/handlers/webhooks/webhooks_test.go

@@ -1,5 +1,4 @@
 //go:build test
-// +build test
 
 package webhooks
 
@@ -194,7 +193,7 @@ func TestHandleSquareWebhook_BodyTooLarge(t *testing.T) {
 }
 
 func TestHandleSquareWebhook_ValidSignatureWithEnvKey(t *testing.T) {
-	
+
 	body := []byte(`{"type":"payment.updated","event_id":"evt_1"}`)
 	key := "env-signing-key"
 	notificationURL := "http://localhost:8080/webhooks/square"
@@ -213,7 +212,7 @@ func TestHandleSquareWebhook_ValidSignatureWithEnvKey(t *testing.T) {
 }
 
 func TestHandleSquareWebhook_InvalidSignatureWithEnvKey(t *testing.T) {
-	
+
 	body := []byte(`{"type":"payment.updated","event_id":"evt_1"}`)
 
 	t.Setenv("SQUARE_WEBHOOK_SIGNATURE_KEY", "env-signing-key")
@@ -225,7 +224,7 @@ func TestHandleSquareWebhook_InvalidSignatureWithEnvKey(t *testing.T) {
 }
 
 func TestHandleSquareWebhook_NoSignatureWhenKeySet(t *testing.T) {
-	
+
 	body := []byte(`{"type":"payment.updated","event_id":"evt_1"}`)
 
 	t.Setenv("SQUARE_WEBHOOK_SIGNATURE_KEY", "env-signing-key")
@@ -238,7 +237,7 @@ func TestHandleSquareWebhook_NoSignatureWhenKeySet(t *testing.T) {
 }
 
 func TestHandleSquareWebhook_SignatureSkippedWhenKeyEmpty(t *testing.T) {
-	
+
 	t.Setenv("SQUARE_WEBHOOK_SIGNATURE_KEY", "")
 
 	body := []byte(`{"type":"payment.updated","event_id":"evt_1"}`)