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/session/session_test.go

@@ -173,8 +173,18 @@ func TestSetUser_SetsAllFields(t *testing.T) {
 	)
 }
 
-func TestGetUserID(t *testing.T) {
-	t.Parallel()
+// testSessionGetter exercises a session string getter before and
+// after SetUser: it must report false with an empty value on a
+// fresh session, then true with the expected value once
+// SetUser(sess, "user-xyz", "bob") has run.
+func testSessionGetter(
+	t *testing.T,
+	get func(
+		*session.Session, *sessions.Session,
+	) (string, bool),
+	expected string,
+) {
+	t.Helper()
 
 	s := testSession(t)
 
@@ -185,44 +195,46 @@ func TestGetUserID(t *testing.T) {
 	require.NoError(t, err)
 
 	// Before setting user
-	userID, ok := s.GetUserID(sess)
+	val, ok := get(s, sess)
 	assert.False(
-		t, ok, "should return false when no user ID is set",
+		t, ok, "should return false before SetUser",
 	)
-	assert.Empty(t, userID)
+	assert.Empty(t, val)
 
 	// After setting user
 	s.SetUser(sess, "user-xyz", "bob")
 
-	userID, ok = s.GetUserID(sess)
+	val, ok = get(s, sess)
 	assert.True(t, ok)
-	assert.Equal(t, "user-xyz", userID)
+	assert.Equal(t, expected, val)
+}
+
+func TestGetUserID(t *testing.T) {
+	t.Parallel()
+
+	testSessionGetter(
+		t,
+		func(
+			s *session.Session, sess *sessions.Session,
+		) (string, bool) {
+			return s.GetUserID(sess)
+		},
+		"user-xyz",
+	)
 }
 
 func TestGetUsername(t *testing.T) {
 	t.Parallel()
 
-	s := testSession(t)
-
-	req := httptest.NewRequestWithContext(
-		context.Background(), http.MethodGet, "/", nil)
-
-	sess, err := s.Get(req)
-	require.NoError(t, err)
-
-	// Before setting user
-	username, ok := s.GetUsername(sess)
-	assert.False(
-		t, ok, "should return false when no username is set",
+	testSessionGetter(
+		t,
+		func(
+			s *session.Session, sess *sessions.Session,
+		) (string, bool) {
+			return s.GetUsername(sess)
+		},
+		"bob",
 	)
-	assert.Empty(t, username)
-
-	// After setting user
-	s.SetUser(sess, "user-xyz", "bob")
-
-	username, ok = s.GetUsername(sess)
-	assert.True(t, ok)
-	assert.Equal(t, "bob", username)
 }
 
 // --- IsAuthenticated Tests ---

internal/session/testing.go

@@ -12,7 +12,12 @@ import (
 // middleware and handler tests to use real session functionality. The key
 // parameter is the raw 32-byte authentication key used for session encryption
 // and CSRF cookie signing.
-func NewForTest(store *sessions.CookieStore, cfg *config.Config, log *slog.Logger, key []byte) *Session {
+func NewForTest(
+	store *sessions.CookieStore,
+	cfg *config.Config,
+	log *slog.Logger,
+	key []byte,
+) *Session {
 	return &Session{
 		store:  store,
 		key:    key,