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/delivery/export_test.go

@@ -0,0 +1,240 @@
+package delivery
+
+import (
+	"context"
+	"log/slog"
+	"net"
+	"net/http"
+	"time"
+
+	"gorm.io/gorm"
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// Exported constants for test access.
+const (
+	ExportDeliveryChannelSize     = deliveryChannelSize
+	ExportRetryChannelSize        = retryChannelSize
+	ExportDefaultFailureThreshold = defaultFailureThreshold
+	ExportDefaultCooldown         = defaultCooldown
+)
+
+// ExportIsBlockedIP exposes isBlockedIP for testing.
+func ExportIsBlockedIP(ip net.IP) bool {
+	return isBlockedIP(ip)
+}
+
+// ExportBlockedNetworks exposes blockedNetworks.
+func ExportBlockedNetworks() []*net.IPNet {
+	return blockedNetworks
+}
+
+// ExportIsForwardableHeader exposes isForwardableHeader.
+func ExportIsForwardableHeader(name string) bool {
+	return isForwardableHeader(name)
+}
+
+// ExportTruncate exposes truncate for testing.
+func ExportTruncate(s string, maxLen int) string {
+	return truncate(s, maxLen)
+}
+
+// ExportDeliverHTTP exposes deliverHTTP for testing.
+func (e *Engine) ExportDeliverHTTP(
+	ctx context.Context,
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	task *Task,
+) {
+	e.deliverHTTP(ctx, webhookDB, d, task)
+}
+
+// ExportDeliverDatabase exposes deliverDatabase.
+func (e *Engine) ExportDeliverDatabase(
+	webhookDB *gorm.DB, d *database.Delivery,
+) {
+	e.deliverDatabase(webhookDB, d)
+}
+
+// ExportDeliverLog exposes deliverLog for testing.
+func (e *Engine) ExportDeliverLog(
+	webhookDB *gorm.DB, d *database.Delivery,
+) {
+	e.deliverLog(webhookDB, d)
+}
+
+// ExportDeliverSlack exposes deliverSlack for testing.
+func (e *Engine) ExportDeliverSlack(
+	ctx context.Context,
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+) {
+	e.deliverSlack(ctx, webhookDB, d)
+}
+
+// ExportProcessNewTask exposes processNewTask.
+func (e *Engine) ExportProcessNewTask(
+	ctx context.Context, task *Task,
+) {
+	e.processNewTask(ctx, task)
+}
+
+// ExportProcessRetryTask exposes processRetryTask.
+func (e *Engine) ExportProcessRetryTask(
+	ctx context.Context, task *Task,
+) {
+	e.processRetryTask(ctx, task)
+}
+
+// ExportProcessDelivery exposes processDelivery.
+func (e *Engine) ExportProcessDelivery(
+	ctx context.Context,
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	task *Task,
+) {
+	e.processDelivery(ctx, webhookDB, d, task)
+}
+
+// ExportGetCircuitBreaker exposes getCircuitBreaker.
+func (e *Engine) ExportGetCircuitBreaker(
+	targetID string,
+) *CircuitBreaker {
+	return e.getCircuitBreaker(targetID)
+}
+
+// ExportParseHTTPConfig exposes parseHTTPConfig.
+func (e *Engine) ExportParseHTTPConfig(
+	configJSON string,
+) (*HTTPTargetConfig, error) {
+	return e.parseHTTPConfig(configJSON)
+}
+
+// ExportParseSlackConfig exposes parseSlackConfig.
+func (e *Engine) ExportParseSlackConfig(
+	configJSON string,
+) (*SlackTargetConfig, error) {
+	return e.parseSlackConfig(configJSON)
+}
+
+// ExportDoHTTPRequest exposes doHTTPRequest.
+func (e *Engine) ExportDoHTTPRequest(
+	ctx context.Context,
+	cfg *HTTPTargetConfig,
+	event *database.Event,
+) (int, string, int64, error) {
+	return e.doHTTPRequest(ctx, cfg, event)
+}
+
+// ExportScheduleRetry exposes scheduleRetry.
+func (e *Engine) ExportScheduleRetry(
+	task Task, delay time.Duration,
+) {
+	e.scheduleRetry(task, delay)
+}
+
+// ExportRecoverPendingDeliveries exposes
+// recoverPendingDeliveries.
+func (e *Engine) ExportRecoverPendingDeliveries(
+	ctx context.Context,
+	webhookDB *gorm.DB,
+	webhookID string,
+) {
+	e.recoverPendingDeliveries(
+		ctx, webhookDB, webhookID,
+	)
+}
+
+// ExportRecoverWebhookDeliveries exposes
+// recoverWebhookDeliveries.
+func (e *Engine) ExportRecoverWebhookDeliveries(
+	ctx context.Context, webhookID string,
+) {
+	e.recoverWebhookDeliveries(ctx, webhookID)
+}
+
+// ExportRecoverInFlight exposes recoverInFlight.
+func (e *Engine) ExportRecoverInFlight(
+	ctx context.Context,
+) {
+	e.recoverInFlight(ctx)
+}
+
+// ExportStart exposes start for testing.
+func (e *Engine) ExportStart(ctx context.Context) {
+	e.start(ctx)
+}
+
+// ExportStop exposes stop for testing.
+func (e *Engine) ExportStop() {
+	e.stop()
+}
+
+// ExportDeliveryCh returns the delivery channel.
+func (e *Engine) ExportDeliveryCh() chan Task {
+	return e.deliveryCh
+}
+
+// ExportRetryCh returns the retry channel.
+func (e *Engine) ExportRetryCh() chan Task {
+	return e.retryCh
+}
+
+// NewTestEngine creates an Engine for unit tests without
+// database dependencies.
+func NewTestEngine(
+	log *slog.Logger,
+	client *http.Client,
+	workers int,
+) *Engine {
+	return &Engine{
+		log:        log,
+		client:     client,
+		deliveryCh: make(chan Task, deliveryChannelSize),
+		retryCh:    make(chan Task, retryChannelSize),
+		workers:    workers,
+	}
+}
+
+// NewTestEngineSmallRetry creates an Engine with a tiny
+// retry channel buffer for overflow testing.
+func NewTestEngineSmallRetry(
+	log *slog.Logger,
+) *Engine {
+	return &Engine{
+		log:     log,
+		retryCh: make(chan Task, 1),
+	}
+}
+
+// NewTestEngineWithDB creates an Engine with a real
+// database and dbManager for integration tests.
+func NewTestEngineWithDB(
+	db *database.Database,
+	dbMgr *database.WebhookDBManager,
+	log *slog.Logger,
+	client *http.Client,
+	workers int,
+) *Engine {
+	return &Engine{
+		database:   db,
+		dbManager:  dbMgr,
+		log:        log,
+		client:     client,
+		deliveryCh: make(chan Task, deliveryChannelSize),
+		retryCh:    make(chan Task, retryChannelSize),
+		workers:    workers,
+	}
+}
+
+// NewTestCircuitBreaker creates a CircuitBreaker with
+// custom settings for testing.
+func NewTestCircuitBreaker(
+	threshold int, cooldown time.Duration,
+) *CircuitBreaker {
+	return &CircuitBreaker{
+		state:     CircuitClosed,
+		threshold: threshold,
+		cooldown:  cooldown,
+	}
+}