refactor: use pinned golangci-lint Docker image for linting

Refactor Dockerfile to use a separate lint stage with a pinned
golangci-lint v2.11.3 Docker image instead of installing
golangci-lint via curl in the builder stage. This follows the
pattern used by sneak/pixa.

Changes:
- Dockerfile: separate lint stage using golangci/golangci-lint:v2.11.3
  (Debian-based, pinned by sha256) with COPY --from=lint dependency
- Bump Go from 1.24 to 1.26.1 (golang:1.26.1-bookworm, pinned)
- Bump golangci-lint from v1.64.8 to v2.11.3
- Migrate .golangci.yml from v1 to v2 format (same linters, format only)
- All Docker images pinned by sha256 digest
- Fix all lint issues from the v2 linter upgrade:
  - Add package comments to all packages
  - Add doc comments to all exported types, functions, and methods
  - Fix unchecked errors (errcheck)
  - Fix unused parameters (revive)
  - Fix gosec warnings (MaxBytesReader for form parsing)
  - Fix staticcheck suggestions (fmt.Fprintf instead of WriteString)
  - Rename DeliveryTask to Task to avoid stutter (delivery.Task)
  - Rename shadowed builtin 'max' parameter
- Update README.md version requirements

internal/globals/globals_test.go

@@ -1,26 +1,30 @@
-package globals
+package globals_test
 
 import (
 	"testing"
 
-	"go.uber.org/fx/fxtest"
+	"sneak.berlin/go/webhooker/internal/globals"
 )
 
-func TestNew(t *testing.T) {
-	// Set test values
-	Appname = "test-app"
-	Version = "1.0.0"
+func TestGlobalsFields(t *testing.T) {
+	t.Parallel()
 
-	lc := fxtest.NewLifecycle(t)
-	globals, err := New(lc)
-	if err != nil {
-		t.Fatalf("New() error = %v", err)
+	g := &globals.Globals{
+		Appname: "test-app",
+		Version: "1.0.0",
 	}
 
-	if globals.Appname != "test-app" {
-		t.Errorf("Appname = %v, want %v", globals.Appname, "test-app")
+	if g.Appname != "test-app" {
+		t.Errorf(
+			"Appname = %v, want %v",
+			g.Appname, "test-app",
+		)
 	}
-	if globals.Version != "1.0.0" {
-		t.Errorf("Version = %v, want %v", globals.Version, "1.0.0")
+
+	if g.Version != "1.0.0" {
+		t.Errorf(
+			"Version = %v, want %v",
+			g.Version, "1.0.0",
+		)
 	}
 }