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/database_test.go

@@ -11,6 +11,15 @@ import (
 	"sneak.berlin/go/webhooker/internal/logger"
 )
 
+const (
+	// testAppname is the Globals.Appname used in tests.
+	testAppname = "webhooker-test"
+	// testVersion is the Globals.Version used in tests.
+	testVersion = "test"
+	// testContentType is the event content type used in tests.
+	testContentType = "application/json"
+)
+
 func setupTestDB(
 	t *testing.T,
 ) (*database.Database, *fxtest.Lifecycle) {
@@ -19,8 +28,8 @@ func setupTestDB(
 	lc := fxtest.NewLifecycle(t)
 
 	g := &globals.Globals{
-		Appname: "webhooker-test",
-		Version: "test",
+		Appname: testAppname,
+		Version: testVersion,
 	}
 
 	l, err := logger.New(