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/server/server.go

@@ -1,3 +1,5 @@
+// Package server wires up HTTP routes and manages the
+// application lifecycle.
 package server
 
 import (
@@ -21,9 +23,20 @@ import (
 	"github.com/go-chi/chi"
 )
 
-// nolint:revive // ServerParams is a standard fx naming convention
+const (
+	// shutdownTimeout is the maximum time to wait for the HTTP
+	// server to finish in-flight requests during shutdown.
+	shutdownTimeout = 5 * time.Second
+
+	// sentryFlushTimeout is the maximum time to wait for Sentry
+	// to flush pending events during shutdown.
+	sentryFlushTimeout = 2 * time.Second
+)
+
+//nolint:revive // ServerParams is a standard fx naming convention.
 type ServerParams struct {
 	fx.In
+
 	Logger     *logger.Logger
 	Globals    *globals.Globals
 	Config     *config.Config
@@ -31,12 +44,13 @@ type ServerParams struct {
 	Handlers   *handlers.Handlers
 }
 
+// Server is the main HTTP server that wires up routes and manages
+// graceful shutdown.
 type Server struct {
 	startupTime   time.Time
 	exitCode      int
 	sentryEnabled bool
 	log           *slog.Logger
-	ctx           context.Context
 	cancelFunc    context.CancelFunc
 	httpServer    *http.Server
 	router        *chi.Mux
@@ -45,6 +59,8 @@ type Server struct {
 	h             *handlers.Handlers
 }
 
+// New creates a Server that starts the HTTP listener on fx start
+// and stops it gracefully.
 func New(lc fx.Lifecycle, params ServerParams) (*Server, error) {
 	s := new(Server)
 	s.params = params
@@ -53,19 +69,23 @@ func New(lc fx.Lifecycle, params ServerParams) (*Server, error) {
 	s.log = params.Logger.Get()
 
 	lc.Append(fx.Hook{
-		OnStart: func(ctx context.Context) error {
+		OnStart: func(_ context.Context) error {
 			s.startupTime = time.Now()
 			go s.Run()
+
 			return nil
 		},
 		OnStop: func(ctx context.Context) error {
-			s.cleanShutdown()
+			s.cleanShutdown(ctx)
+
 			return nil
 		},
 	})
+
 	return s, nil
 }
 
+// Run configures Sentry and starts serving HTTP requests.
 func (s *Server) Run() {
 	s.configure()
 
@@ -75,6 +95,12 @@ func (s *Server) Run() {
 	s.serve()
 }
 
+// MaintenanceMode returns whether the server is in maintenance
+// mode.
+func (s *Server) MaintenanceMode() bool {
+	return s.params.Config.MaintenanceMode
+}
+
 func (s *Server) enableSentry() {
 	s.sentryEnabled = false
 
@@ -83,29 +109,37 @@ func (s *Server) enableSentry() {
 	}
 
 	err := sentry.Init(sentry.ClientOptions{
-		Dsn:     s.params.Config.SentryDSN,
-		Release: fmt.Sprintf("%s-%s", s.params.Globals.Appname, s.params.Globals.Version),
+		Dsn: s.params.Config.SentryDSN,
+		Release: fmt.Sprintf(
+			"%s-%s",
+			s.params.Globals.Appname,
+			s.params.Globals.Version,
+		),
 	})
 	if err != nil {
 		s.log.Error("sentry init failure", "error", err)
 		// Don't use fatal since we still want the service to run
 		return
 	}
+
 	s.log.Info("sentry error reporting activated")
 	s.sentryEnabled = true
 }
 
 func (s *Server) serve() int {
-	s.ctx, s.cancelFunc = context.WithCancel(context.Background())
+	ctx, cancelFunc := context.WithCancel(context.Background())
+	s.cancelFunc = cancelFunc
 
 	// signal watcher
 	go func() {
 		c := make(chan os.Signal, 1)
+
 		signal.Ignore(syscall.SIGPIPE)
 		signal.Notify(c, os.Interrupt, syscall.SIGTERM)
 		// block and wait for signal
 		sig := <-c
 		s.log.Info("signal received", "signal", sig.String())
+
 		if s.cancelFunc != nil {
 			// cancelling the main context will trigger a clean
 			// shutdown via the fx OnStop hook.
@@ -115,9 +149,9 @@ func (s *Server) serve() int {
 
 	go s.serveUntilShutdown()
 
-	<-s.ctx.Done()
+	<-ctx.Done()
 	// Shutdown is handled by the fx OnStop hook (cleanShutdown).
-	// Do not call cleanShutdown() here to avoid a double invocation.
+	// Do not call cleanShutdown() here to avoid double invocation.
 	return s.exitCode
 }
 
@@ -125,27 +159,29 @@ func (s *Server) cleanupForExit() {
 	s.log.Info("cleaning up")
 }
 
-func (s *Server) cleanShutdown() {
+func (s *Server) cleanShutdown(ctx context.Context) {
 	// initiate clean shutdown
 	s.exitCode = 0
-	ctxShutdown, shutdownCancel := context.WithTimeout(context.Background(), 5*time.Second)
+
+	ctxShutdown, shutdownCancel := context.WithTimeout(
+		ctx, shutdownTimeout,
+	)
 	defer shutdownCancel()
 
-	if err := s.httpServer.Shutdown(ctxShutdown); err != nil {
-		s.log.Error("server clean shutdown failed", "error", err)
+	err := s.httpServer.Shutdown(ctxShutdown)
+	if err != nil {
+		s.log.Error(
+			"server clean shutdown failed", "error", err,
+		)
 	}
 
 	s.cleanupForExit()
 
 	if s.sentryEnabled {
-		sentry.Flush(2 * time.Second)
+		sentry.Flush(sentryFlushTimeout)
 	}
 }
 
-func (s *Server) MaintenanceMode() bool {
-	return s.params.Config.MaintenanceMode
-}
-
 func (s *Server) configure() {
 	// identify ourselves in the logs
 	s.params.Logger.Identify()