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/middleware/middleware_test.go

@@ -484,8 +484,13 @@ func metricsAuthMiddleware(
 	return middleware.NewForTest(log, cfg, sessManager)
 }
 
-func TestMetricsAuth_ValidCredentials(t *testing.T) {
-	t.Parallel()
+// runMetricsAuthRequest sends a GET /metrics request with the
+// given basic-auth password through MetricsAuth and reports
+// whether the wrapped handler ran plus the recorded response.
+func runMetricsAuthRequest(
+	t *testing.T, password string,
+) (bool, *httptest.ResponseRecorder) {
+	t.Helper()
 
 	m := metricsAuthMiddleware(t)
 
@@ -503,12 +508,20 @@ func TestMetricsAuth_ValidCredentials(t *testing.T) {
 		context.Background(),
 		http.MethodGet, "/metrics", nil,
 	)
-	req.SetBasicAuth("admin", "secret")
+	req.SetBasicAuth("admin", password)
 
 	w := httptest.NewRecorder()
 
 	handler.ServeHTTP(w, req)
 
+	return called, w
+}
+
+func TestMetricsAuth_ValidCredentials(t *testing.T) {
+	t.Parallel()
+
+	called, w := runMetricsAuthRequest(t, "secret")
+
 	assert.True(
 		t, called,
 		"handler should be called with valid basic auth",
@@ -519,27 +532,7 @@ func TestMetricsAuth_ValidCredentials(t *testing.T) {
 func TestMetricsAuth_InvalidCredentials(t *testing.T) {
 	t.Parallel()
 
-	m := metricsAuthMiddleware(t)
-
-	var called bool
-
-	handler := m.MetricsAuth()(http.HandlerFunc(
-		func(w http.ResponseWriter, _ *http.Request) {
-			called = true
-
-			w.WriteHeader(http.StatusOK)
-		},
-	))
-
-	req := httptest.NewRequestWithContext(
-		context.Background(),
-		http.MethodGet, "/metrics", nil,
-	)
-	req.SetBasicAuth("admin", "wrong-password")
-
-	w := httptest.NewRecorder()
-
-	handler.ServeHTTP(w, req)
+	called, w := runMetricsAuthRequest(t, "wrong-password")
 
 	assert.False(
 		t, called,