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/database/database_test.go

@@ -1,4 +1,4 @@
-package database
+package database_test
 
 import (
 	"context"
@@ -6,37 +6,37 @@ import (
 
 	"go.uber.org/fx/fxtest"
 	"sneak.berlin/go/webhooker/internal/config"
+	"sneak.berlin/go/webhooker/internal/database"
 	"sneak.berlin/go/webhooker/internal/globals"
 	"sneak.berlin/go/webhooker/internal/logger"
 )
 
-func TestDatabaseConnection(t *testing.T) {
-	// Set up test dependencies
+func setupTestDB(
+	t *testing.T,
+) (*database.Database, *fxtest.Lifecycle) {
+	t.Helper()
+
 	lc := fxtest.NewLifecycle(t)
 
-	// Create globals
-	globals.Appname = "webhooker-test"
-	globals.Version = "test"
-
-	g, err := globals.New(lc)
-	if err != nil {
-		t.Fatalf("Failed to create globals: %v", err)
+	g := &globals.Globals{
+		Appname: "webhooker-test",
+		Version: "test",
 	}
 
-	// Create logger
-	l, err := logger.New(lc, logger.LoggerParams{Globals: g})
+	l, err := logger.New(
+		lc,
+		logger.LoggerParams{Globals: g},
+	)
 	if err != nil {
 		t.Fatalf("Failed to create logger: %v", err)
 	}
 
-	// Create config with DataDir pointing to a temp directory
 	c := &config.Config{
 		DataDir:     t.TempDir(),
 		Environment: "dev",
 	}
 
-	// Create database
-	db, err := New(lc, DatabaseParams{
+	db, err := database.New(lc, database.DatabaseParams{
 		Config: c,
 		Logger: l,
 	})
@@ -44,31 +44,45 @@ func TestDatabaseConnection(t *testing.T) {
 		t.Fatalf("Failed to create database: %v", err)
 	}
 
-	// Start lifecycle (this will trigger the connection)
+	return db, lc
+}
+
+func TestDatabaseConnection(t *testing.T) {
+	t.Parallel()
+
+	db, lc := setupTestDB(t)
 	ctx := context.Background()
-	err = lc.Start(ctx)
+
+	err := lc.Start(ctx)
 	if err != nil {
 		t.Fatalf("Failed to connect to database: %v", err)
 	}
+
 	defer func() {
-		if stopErr := lc.Stop(ctx); stopErr != nil {
-			t.Errorf("Failed to stop lifecycle: %v", stopErr)
+		stopErr := lc.Stop(ctx)
+		if stopErr != nil {
+			t.Errorf(
+				"Failed to stop lifecycle: %v",
+				stopErr,
+			)
 		}
 	}()
 
-	// Verify we can get the DB instance
 	if db.DB() == nil {
 		t.Error("Expected non-nil database connection")
 	}
 
-	// Test that we can perform a simple query
 	var result int
+
 	err = db.DB().Raw("SELECT 1").Scan(&result).Error
 	if err != nil {
 		t.Fatalf("Failed to execute test query: %v", err)
 	}
 
 	if result != 1 {
-		t.Errorf("Expected query result to be 1, got %d", result)
+		t.Errorf(
+			"Expected query result to be 1, got %d",
+			result,
+		)
 	}
 }