refactor: use pinned golangci-lint Docker image for linting (#55)

Closes [issue #50](#50)

## Summary

Refactors the Dockerfile to use a separate lint stage with a pinned golangci-lint Docker image, following the pattern used by [sneak/pixa](https://git.eeqj.de/sneak/pixa). This replaces the previous approach of installing golangci-lint via curl in the builder stage.

## Changes

### Dockerfile
- **New `lint` stage** using `golangci/golangci-lint:v2.11.3` (Debian-based, pinned by sha256 digest) as a separate build stage
- **Builder stage** depends on lint via `COPY --from=lint /src/go.sum /dev/null` — build won't proceed unless linting passes
- **Go bumped** from 1.24 to 1.26.1 (`golang:1.26.1-bookworm`, pinned by sha256)
- **golangci-lint bumped** from v1.64.8 to v2.11.3
- All three Docker images (golangci-lint, golang, alpine) pinned by sha256 digest
- Debian-based golangci-lint image used (not Alpine) because mattn/go-sqlite3 CGO does not compile on musl (off64_t)

### Linter Config (.golangci.yml)
- Migrated from v1 to v2 format (`version: "2"` added)
- Removed linters no longer available in v2: `gofmt` (handled by `make fmt-check`), `gosimple` (merged into `staticcheck`), `typecheck` (always-on in v2)
- Same set of linters enabled — no rules weakened

### Code Fixes (all lint issues from v2 upgrade)
- Added package comments to all packages
- Added doc comments to all exported types, functions, and methods
- Fixed unchecked errors flagged by `errcheck` (sqlDB.Close, os.Setenv in tests, resp.Body.Close, fmt.Fprint)
- Fixed unused parameters flagged by `revive` (renamed to `_`)
- Fixed `gosec` G120 warnings: added `http.MaxBytesReader` before `r.ParseForm()` calls
- Fixed `staticcheck` QF1012: replaced `WriteString(fmt.Sprintf(...))` with `fmt.Fprintf`
- Fixed `staticcheck` QF1003: converted if/else chain to tagged switch
- Renamed `DeliveryTask` → `Task` to avoid package stutter (`delivery.Task` instead of `delivery.DeliveryTask`)
- Renamed shadowed builtin `max` parameter to `upperBound` in `cryptoRandInt`
- Used `t.Setenv` instead of `os.Setenv` in tests (auto-restores)

### README.md
- Updated version requirements: Go 1.26+, golangci-lint v2.11+
- Updated Dockerfile description in project structure

## Verification

`docker build .` passes cleanly — formatting check, linting, all tests, and build all succeed.

Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #55
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/logger/logger.go

@@ -1,3 +1,5 @@
+// Package logger provides structured logging with dynamic level
+// control.
 package logger
 
 import (
@@ -10,19 +12,25 @@ 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
@@ -37,17 +45,22 @@ 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 {
 				return slog.Time(slog.TimeKey, t.UTC())
 			}
+
+			return a
 		}
+
 		return a
 	}
 
 	var handler slog.Handler
+
 	opts := &slog.HandlerOptions{
 		Level:       l.levelVar,
 		ReplaceAttr: replaceAttr,
@@ -69,15 +82,18 @@ 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 +101,8 @@ 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
 }