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/delivery/engine_test.go

@@ -27,6 +27,9 @@ import (
 	"sneak.berlin/go/webhooker/internal/delivery"
 )
 
+// testContentType is the event content type used in tests.
+const testContentType = "application/json"
+
 func testWebhookDB(t *testing.T) *gorm.DB {
 	t.Helper()
 
@@ -94,10 +97,10 @@ func seedEvent(
 	event := database.Event{
 		WebhookID:    uuid.New().String(),
 		EntrypointID: uuid.New().String(),
-		Method:       "POST",
+		Method:       http.MethodPost,
 		Headers:      `{"Content-Type":["application/json"]}`,
 		Body:         body,
-		ContentType:  "application/json",
+		ContentType:  testContentType,
 	}
 
 	require.NoError(t, db.Create(&event).Error)
@@ -1113,10 +1116,10 @@ func TestDoHTTPRequest_ForwardsHeaders(t *testing.T) {
 	}
 
 	event := &database.Event{
-		Method:      "POST",
+		Method:      http.MethodPost,
 		Headers:     `{"X-Custom":["value1"],"Content-Type":["application/json"]}`,
 		Body:        `{"test":true}`,
-		ContentType: "application/json",
+		ContentType: testContentType,
 	}
 
 	statusCode, _, _, err := e.ExportDoHTTPRequest(
@@ -1138,7 +1141,7 @@ func TestDoHTTPRequest_ForwardsHeaders(t *testing.T) {
 	)
 
 	assert.Equal(t,
-		"application/json",
+		testContentType,
 		receivedHeaders.Get("Content-Type"),
 	)
 
@@ -1297,8 +1300,8 @@ func TestFormatSlackMessage_JSONBody(t *testing.T) {
 	t.Parallel()
 
 	event := &database.Event{
-		Method:      "POST",
-		ContentType: "application/json",
+		Method:      http.MethodPost,
+		ContentType: testContentType,
 		Body: `{"action":"push",` +
 			`"repo":"test/repo",` +
 			`"ref":"refs/heads/main"}`,
@@ -1323,7 +1326,7 @@ func TestFormatSlackMessage_NonJSONBody(t *testing.T) {
 	t.Parallel()
 
 	event := &database.Event{
-		Method:      "POST",
+		Method:      http.MethodPost,
 		ContentType: "text/plain",
 		Body:        "hello world plain text",
 	}
@@ -1346,8 +1349,8 @@ func TestFormatSlackMessage_EmptyBody(t *testing.T) {
 	t.Parallel()
 
 	event := &database.Event{
-		Method:      "POST",
-		ContentType: "application/json",
+		Method:      http.MethodPost,
+		ContentType: testContentType,
 		Body:        "",
 	}
 	event.CreatedAt = time.Date(
@@ -1375,8 +1378,8 @@ func TestFormatSlackMessage_LargeJSONTruncated(
 	require.NoError(t, err)
 
 	event := &database.Event{
-		Method:      "POST",
-		ContentType: "application/json",
+		Method:      http.MethodPost,
+		ContentType: testContentType,
 		Body:        string(largeJSON),
 	}
 	event.CreatedAt = time.Date(
@@ -1705,7 +1708,7 @@ func assertLogLineComplete(
 		"log line must contain the webhook id",
 	)
 
-	assert.Contains(t, out, "application/json",
+	assert.Contains(t, out, testContentType,
 		"log line must contain the content type",
 	)
 }