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/database/retention_test.go

@@ -2,6 +2,7 @@ package database_test
 
 import (
 	"context"
+	"net/http"
 	"testing"
 	"time"
 
@@ -30,8 +31,8 @@ func setupRetentionTest(t *testing.T) *retentionTestEnv {
 	lc := fxtest.NewLifecycle(t)
 
 	g := &globals.Globals{
-		Appname: "webhooker-test",
-		Version: "test",
+		Appname: testAppname,
+		Version: testVersion,
 	}
 
 	l, err := logger.New(lc, logger.LoggerParams{Globals: g})
@@ -117,9 +118,9 @@ func seedEventChain(
 	event := &database.Event{
 		WebhookID:    webhookID,
 		EntrypointID: uuid.New().String(),
-		Method:       "POST",
+		Method:       http.MethodPost,
 		Body:         `{"seed": true}`,
-		ContentType:  "application/json",
+		ContentType:  testContentType,
 	}
 	event.CreatedAt = createdAt
 	require.NoError(t, db.Create(event).Error)