Update golangci-lint to v2.12.2 with canonical config (#86)

Bumps golangci-lint from v2.11.3 to v2.12.2 and adopts the canonical lint config.

## Version pins

- `Dockerfile`: `golangci/golangci-lint:v2.12.2` Debian image, pinned by digest, dated `2026-08-07`
- `script/bootstrap`: `GOLANGCI_LINT_VERSION=2.12.2` with updated sha256 pins for the `linux-amd64` and `linux-arm64` release archives

## Config

`.golangci.yml` replaced with the canonical config. The previous file kept `lll`/`funlen`/`cyclop`/`dupl` settings under the top-level `linters-settings` key, which the v2 schema ignores; the canonical config nests them under `linters.settings`, so those thresholds now actually apply. The unsupported `issues.exclude-use-default` key was dropped.

## Lint fixes (32 findings)

- `lll` (7): wrapped or shortened over-length lines (struct tag comments moved above fields, test logger construction split, `session.NewForTest` signature wrapped, shortened a `#nosec` comment)
- `goconst` (17): replaced repeated `"POST"`/`"PUT"` literals with `http.MethodPost`/`http.MethodPut`, added shared test constants for `webhooker-test`/`test`/`application/json`, and added `tmplKeyError`/`tmplKeyWebhook` constants for template data keys in `internal/handlers`
- `dupl` (8): merged `buildHTTPTargetConfig` and `buildSlackTargetConfig` into a parameterized `buildURLTargetConfig`; removed the duplicate `iWebhookDB` test helper in favor of `testWebhookDB`; extracted shared helpers in middleware and session tests

No `//nolint` directives were added and behavior is unchanged. `make check` (fmt-check, tests, lint) passes.

Note: golangci-lint v2.12 deprecates the `gomodguard` linter in favor of `gomodguard_v2`; the canonical config change for that is left for a future coordinated update.
Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #86
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

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(