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/logger/logger.go

@@ -1,3 +1,4 @@
+// Package logger provides structured logging with dynamic level control.
 package logger
 
 import (
@@ -10,19 +11,22 @@ import (
 	"sneak.berlin/go/webhooker/internal/globals"
 )
 
-// nolint:revive // LoggerParams is a standard fx naming convention
+//nolint:revive // LoggerParams is a standard fx naming convention.
 type LoggerParams struct {
 	fx.In
 	Globals *globals.Globals
 }
 
+// Logger wraps slog with dynamic level control and structured output.
 type Logger struct {
 	logger   *slog.Logger
 	levelVar *slog.LevelVar
 	params   LoggerParams
 }
 
-// nolint:revive // lc parameter is required by fx even if unused
+// New creates a Logger that outputs text (TTY) or JSON (non-TTY) to stdout.
+//
+//nolint:revive // lc parameter is required by fx even if unused.
 func New(lc fx.Lifecycle, params LoggerParams) (*Logger, error) {
 	l := new(Logger)
 	l.params = params
@@ -37,7 +41,8 @@ func New(lc fx.Lifecycle, params LoggerParams) (*Logger, error) {
 		tty = true
 	}
 
-	replaceAttr := func(_ []string, a slog.Attr) slog.Attr { // nolint:revive // groups unused
+	//nolint:revive // groups param unused but required by slog ReplaceAttr signature.
+	replaceAttr := func(_ []string, a slog.Attr) slog.Attr {
 		// Always use UTC for timestamps
 		if a.Key == slog.TimeKey {
 			if t, ok := a.Value.Any().(time.Time); ok {
@@ -69,15 +74,18 @@ func New(lc fx.Lifecycle, params LoggerParams) (*Logger, error) {
 	return l, nil
 }
 
+// EnableDebugLogging switches the log level to debug.
 func (l *Logger) EnableDebugLogging() {
 	l.levelVar.Set(slog.LevelDebug)
 	l.logger.Debug("debug logging enabled", "debug", true)
 }
 
+// Get returns the underlying slog.Logger.
 func (l *Logger) Get() *slog.Logger {
 	return l.logger
 }
 
+// Identify logs the application name and version at startup.
 func (l *Logger) Identify() {
 	l.logger.Info("starting",
 		"appname", l.params.Globals.Appname,
@@ -85,7 +93,7 @@ func (l *Logger) Identify() {
 	)
 }
 
-// Helper methods to maintain compatibility with existing code
+// Writer returns an io.Writer suitable for standard library loggers.
 func (l *Logger) Writer() io.Writer {
 	return os.Stdout
 }