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/handlers/handlers.go

@@ -1,3 +1,4 @@
+// Package handlers provides HTTP request handlers for the webhooker web UI and API.
 package handlers
 
 import (
@@ -18,7 +19,7 @@ import (
 	"sneak.berlin/go/webhooker/templates"
 )
 
-// nolint:revive // HandlersParams is a standard fx naming convention
+//nolint:revive // HandlersParams is a standard fx naming convention.
 type HandlersParams struct {
 	fx.In
 	Logger       *logger.Logger
@@ -30,6 +31,7 @@ type HandlersParams struct {
 	Notifier     delivery.Notifier
 }
 
+// Handlers provides HTTP handler methods for all application routes.
 type Handlers struct {
 	params    *HandlersParams
 	log       *slog.Logger
@@ -53,6 +55,7 @@ func parsePageTemplate(pageFile string) *template.Template {
 	)
 }
 
+// New creates a Handlers instance, parsing all page templates at startup.
 func New(lc fx.Lifecycle, params HandlersParams) (*Handlers, error) {
 	s := new(Handlers)
 	s.params = &params
@@ -76,15 +79,15 @@ func New(lc fx.Lifecycle, params HandlersParams) (*Handlers, error) {
 	}
 
 	lc.Append(fx.Hook{
-		OnStart: func(ctx context.Context) error {
+		OnStart: func(_ context.Context) error {
 			return nil
 		},
 	})
 	return s, nil
 }
 
-//nolint:unparam // r parameter will be used in the future for request context
-func (s *Handlers) respondJSON(w http.ResponseWriter, r *http.Request, data interface{}, status int) {
+//nolint:unparam // r parameter will be used in the future for request context.
+func (s *Handlers) respondJSON(w http.ResponseWriter, _ *http.Request, data interface{}, status int) {
 	w.Header().Set("Content-Type", "application/json")
 	w.WriteHeader(status)
 	if data != nil {
@@ -95,8 +98,8 @@ func (s *Handlers) respondJSON(w http.ResponseWriter, r *http.Request, data inte
 	}
 }
 
-//nolint:unparam,unused // will be used for handling JSON requests
-func (s *Handlers) decodeJSON(w http.ResponseWriter, r *http.Request, v interface{}) error {
+//nolint:unparam,unused // will be used for handling JSON requests.
+func (s *Handlers) decodeJSON(_ http.ResponseWriter, r *http.Request, v interface{}) error {
 	return json.NewDecoder(r.Body).Decode(v)
 }