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/auth.go

@@ -19,7 +19,7 @@ func (h *Handlers) HandleLoginPage() http.HandlerFunc {
 
 		// Render login page
 		data := map[string]any{
-			"Error": "",
+			tmplKeyError: "",
 		}
 
 		h.renderTemplate(w, r, "login.html", data)
@@ -86,7 +86,7 @@ func (h *Handlers) renderLoginError(
 	status int,
 ) {
 	data := map[string]any{
-		"Error": msg,
+		tmplKeyError: msg,
 	}
 
 	w.WriteHeader(status)