Enforce Go style rules across codebase

Apply five style rules: convert iota string enums to typed
string constants, replace errors.As with errors.AsType,
merge three-group imports into two groups, fix multiline
parameter/argument formatting, and replace fmt.Sprintf URL
construction with net/url.

Signed-off-by: Émile Ré <emile@probo.com>

pkg/webhook/sender.go

@@ -170,10 +170,13 @@ func (s *Sender) claimNextWebhookData(ctx context.Context) (*coredata.WebhookDat
 				return fmt.Errorf("cannot insert webhook event: %w", err)
 			}
 
-			deliveries = append(deliveries, pendingDelivery{
-				Event:  event,
-				Config: config,
-			})
+			deliveries = append(
+				deliveries,
+				pendingDelivery{
+					Event:  event,
+					Config: config,
+				},
+			)
 		}
 
 		webhookData.ProcessedAt = &now