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/handlers/handlers.go

@@ -30,6 +30,11 @@ const (
 	defaultRetentionDays = 30
 	// paginationPerPage is the number of items per page.
 	paginationPerPage = 25
+
+	// tmplKeyError is the template data key for an error message.
+	tmplKeyError = "Error"
+	// tmplKeyWebhook is the template data key for a webhook.
+	tmplKeyWebhook = "Webhook"
 )
 
 // errInvalidPassword is returned when a password does not match.