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/http.go

@@ -1,18 +1,33 @@
 package server
 
 import (
+	"errors"
 	"fmt"
 	"net/http"
 	"time"
 )
 
+const (
+	// httpReadTimeout is the maximum duration for reading the
+	// entire request, including the body.
+	httpReadTimeout = 10 * time.Second
+
+	// httpWriteTimeout is the maximum duration before timing out
+	// writes of the response.
+	httpWriteTimeout = 10 * time.Second
+
+	// httpMaxHeaderBytes is the maximum number of bytes the
+	// server will read parsing the request headers.
+	httpMaxHeaderBytes = 1 << 20
+)
+
 func (s *Server) serveUntilShutdown() {
 	listenAddr := fmt.Sprintf(":%d", s.params.Config.Port)
 	s.httpServer = &http.Server{
 		Addr:           listenAddr,
-		ReadTimeout:    10 * time.Second,
-		WriteTimeout:   10 * time.Second,
-		MaxHeaderBytes: 1 << 20,
+		ReadTimeout:    httpReadTimeout,
+		WriteTimeout:   httpWriteTimeout,
+		MaxHeaderBytes: httpMaxHeaderBytes,
 		Handler:        s,
 	}
 
@@ -21,14 +36,21 @@ func (s *Server) serveUntilShutdown() {
 	s.SetupRoutes()
 
 	s.log.Info("http begin listen", "listenaddr", listenAddr)
-	if err := s.httpServer.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+
+	err := s.httpServer.ListenAndServe()
+	if err != nil && !errors.Is(err, http.ErrServerClosed) {
 		s.log.Error("listen error", "error", err)
+
 		if s.cancelFunc != nil {
 			s.cancelFunc()
 		}
 	}
 }
 
-func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+// ServeHTTP delegates to the router.
+func (s *Server) ServeHTTP(
+	w http.ResponseWriter,
+	r *http.Request,
+) {
 	s.router.ServeHTTP(w, r)
 }