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/database/database_test.go

@@ -11,6 +11,15 @@ import (
 	"sneak.berlin/go/webhooker/internal/logger"
 )
 
+const (
+	// testAppname is the Globals.Appname used in tests.
+	testAppname = "webhooker-test"
+	// testVersion is the Globals.Version used in tests.
+	testVersion = "test"
+	// testContentType is the event content type used in tests.
+	testContentType = "application/json"
+)
+
 func setupTestDB(
 	t *testing.T,
 ) (*database.Database, *fxtest.Lifecycle) {
@@ -19,8 +28,8 @@ func setupTestDB(
 	lc := fxtest.NewLifecycle(t)
 
 	g := &globals.Globals{
-		Appname: "webhooker-test",
-		Version: "test",
+		Appname: testAppname,
+		Version: testVersion,
 	}
 
 	l, err := logger.New(