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.go

@@ -1,25 +1,34 @@
+// Package globals provides build-time variables injected via ldflags.
 package globals
 
 import (
 	"go.uber.org/fx"
 )
 
-// these get populated from main() and copied into the Globals object.
+// Build-time variables populated from main() and copied into the
+// Globals object.
+//
+//nolint:gochecknoglobals // Build-time variables set by main().
 var (
 	Appname string
 	Version string
 )
 
+// Globals holds build-time metadata about the application.
 type Globals struct {
 	Appname string
 	Version string
 }
 
-// nolint:revive // lc parameter is required by fx even if unused
+// New creates a Globals instance from the package-level
+// build-time variables.
+//
+//nolint:revive // lc parameter is required by fx even if unused.
 func New(lc fx.Lifecycle) (*Globals, error) {
 	n := &Globals{
 		Appname: Appname,
 		Version: Version,
 	}
+
 	return n, nil
 }