Update golangci-lint to v2.12.2 with canonical config

Bump the golangci-lint Docker image pin in Dockerfile and the
release-archive sha256 pins in script/bootstrap from 2.11.3 to
2.12.2, and replace .golangci.yml with the canonical config. The
canonical config moves lll/funlen/cyclop/dupl settings from the
top-level linters-settings key (ignored by the v2 schema) to
linters.settings, so those thresholds now actually apply.

Fix all findings the newly applied thresholds surfaced:

- lll: wrap or shorten seven over-length lines (struct tag
  comments, test logger construction, a func signature, and a
  nosec comment)
- goconst: use http.MethodPost/http.MethodPut and new shared
  constants for repeated test strings; add tmplKeyError and
  tmplKeyWebhook constants for template data keys in handlers
- dupl: merge buildHTTPTargetConfig and buildSlackTargetConfig
  into a parameterized buildURLTargetConfig; drop the duplicate
  iWebhookDB test helper in favor of testWebhookDB; extract
  shared helpers in middleware and session tests

internal/delivery/engine_integration_test.go

@@ -126,36 +126,6 @@ func iHTTPConfig(url string) string {
 	return string(data)
 }
 
-func iWebhookDB(t *testing.T) *gorm.DB {
-	t.Helper()
-
-	dbPath := filepath.Join(
-		t.TempDir(), "events-test.db",
-	)
-
-	dsn := fmt.Sprintf(
-		"file:%s?cache=shared&mode=rwc", dbPath,
-	)
-
-	sqlDB, err := sql.Open("sqlite", dsn)
-	require.NoError(t, err)
-
-	t.Cleanup(func() { _ = sqlDB.Close() })
-
-	db, err := gorm.Open(
-		sqlite.Dialector{Conn: sqlDB}, &gorm.Config{},
-	)
-	require.NoError(t, err)
-
-	require.NoError(t, db.AutoMigrate(
-		&database.Event{},
-		&database.Delivery{},
-		&database.DeliveryResult{},
-	))
-
-	return db
-}
-
 func iEngine(
 	t *testing.T, workers int,
 ) *delivery.Engine {
@@ -182,10 +152,10 @@ func iSeedEvent(
 	event := database.Event{
 		WebhookID:    webhookID,
 		EntrypointID: uuid.New().String(),
-		Method:       "POST",
+		Method:       http.MethodPost,
 		Headers:      `{}`,
 		Body:         body,
-		ContentType:  "application/json",
+		ContentType:  testContentType,
 	}
 
 	require.NoError(t, db.Create(&event).Error)
@@ -935,7 +905,7 @@ func TestDeliverHTTP_CustomTargetHeaders(t *testing.T) {
 func TestDeliverHTTP_TargetTimeout(t *testing.T) {
 	t.Parallel()
 
-	db := iWebhookDB(t)
+	db := testWebhookDB(t)
 	e := iEngine(t, 1)
 
 	ts := httptest.NewServer(
@@ -987,10 +957,10 @@ func iSeedEventAndDelivery(
 	event := database.Event{
 		WebhookID:    uuid.New().String(),
 		EntrypointID: uuid.New().String(),
-		Method:       "POST",
+		Method:       http.MethodPost,
 		Headers:      `{"Content-Type":["application/json"]}`,
 		Body:         body,
-		ContentType:  "application/json",
+		ContentType:  testContentType,
 	}
 
 	require.NoError(t, db.Create(&event).Error)
@@ -1067,7 +1037,7 @@ func iAssertResultFailed(
 func TestDeliverHTTP_InvalidConfig(t *testing.T) {
 	t.Parallel()
 
-	db := iWebhookDB(t)
+	db := testWebhookDB(t)
 	e := iEngine(t, 1)
 
 	event, del := iSeedEventAndDelivery(