Refactor delivery targets to a Target interface (closes #77)

Each target TYPE is now an implementation of a Target interface,
dispatched from a registry in processDelivery instead of a type
switch on TargetType. Every target owns its full delivery,
including durable retries.

- Target.Deliver receives the context, the per-webhook DB, the
  Delivery, the attempt Task, and a Scheduler for durable
  re-enqueue (the existing timer + retry queue). The target makes
  one attempt, records the DeliveryResult, updates DeliveryStatus,
  and — for retry targets — decides whether to retry, computes its
  own backoff, gates with its own circuit breaker, and reschedules
  via the Scheduler.
- httpTarget and slackTarget share a retry core (retry, backoff,
  circuit breaker). database and log targets are fire-and-forget.
- Slack retry/breaker is gated on MaxRetries: 0 stays
  fire-and-forget (existing Slack targets unchanged), >0 gets
  retry + backoff + breaker on the shared core.
- The engine keeps only the worker pool, queue/channels, restart
  recovery/sweep, the recordResult/updateDeliveryStatus helpers,
  and ScheduleRetry. Recovery/sweep hand each orphaned retrying
  delivery back to its target to recompute the backoff.
- The log target logs the entire inbound webhook: full body and
  headers, method, content type, and the webhook and entrypoint
  ids (supersedes the smaller log-summary work).
- Task gains EntrypointID, populated in the webhook handler, the
  recovery-task builder, and buildEventFromTask.

Behaviour is preserved: existing delivery tests pass with their
export_test wrappers re-pointed at the new targets; new pure
Deliver tests cover the log full-content output and the gated
Slack retry path.

internal/delivery/target_slack.go

@@ -0,0 +1,299 @@
+package delivery
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+	"time"
+
+	"gorm.io/gorm"
+	"sneak.berlin/go/webhooker/internal/database"
+)
+
+// errMissingWebhookURL is returned when a Slack target config
+// omits its webhook URL.
+var errMissingWebhookURL = errors.New(
+	"webhook_url is required",
+)
+
+// SlackTargetConfig holds configuration for slack target
+// types.
+type SlackTargetConfig struct {
+	WebhookURL string `json:"webhookUrl"`
+}
+
+// slackTarget delivers events to Slack incoming webhooks. It
+// formats the event into a Slack message and posts it as
+// JSON. It shares the retry core with the HTTP target: a
+// MaxRetries of 0 stays single-attempt fire-and-forget
+// (preserving existing Slack targets), while a positive
+// MaxRetries adds backoff and circuit breaking.
+type slackTarget struct {
+	*httpCore
+
+	client *http.Client
+}
+
+// Deliver implements Target.
+func (t *slackTarget) Deliver(
+	ctx context.Context,
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	task *Task,
+	sched Scheduler,
+) {
+	cfg, err := parseSlackConfig(d.Target.Config)
+	if err != nil {
+		t.eng.log.Error(
+			"invalid Slack target config",
+			"target_id", d.TargetID,
+			"error", err,
+		)
+
+		t.failConfig(webhookDB, d, err)
+
+		return
+	}
+
+	msg := FormatSlackMessage(&d.Event)
+
+	payload, err := json.Marshal(
+		map[string]string{"text": msg},
+	)
+	if err != nil {
+		t.eng.log.Error(
+			"failed to marshal Slack payload",
+			"target_id", d.TargetID,
+			"error", err,
+		)
+
+		t.failConfig(webhookDB, d, err)
+
+		return
+	}
+
+	attempt := func() attemptResult {
+		return t.attempt(ctx, cfg, payload)
+	}
+
+	t.deliver(
+		webhookDB, d, task, sched,
+		d.Target.MaxRetries, attempt,
+	)
+}
+
+// failConfig records a first-attempt failure for a delivery
+// that could not be prepared (bad config or unmarshalable
+// payload) and marks it failed.
+func (t *slackTarget) failConfig(
+	webhookDB *gorm.DB,
+	d *database.Delivery,
+	err error,
+) {
+	t.eng.recordResult(
+		webhookDB, d, 1,
+		false, 0, "", err.Error(), 0,
+	)
+
+	t.eng.updateDeliveryStatus(
+		webhookDB, d, database.DeliveryStatusFailed,
+	)
+}
+
+// attempt performs a single Slack POST and derives its
+// outcome, preserving the engine's original semantics: a
+// non-2xx response records an "HTTP <code>" error string and
+// a transport error records a "sending request" error.
+func (t *slackTarget) attempt(
+	ctx context.Context,
+	cfg *SlackTargetConfig,
+	payload []byte,
+) attemptResult {
+	start := time.Now()
+
+	req, err := http.NewRequestWithContext(
+		ctx,
+		http.MethodPost,
+		cfg.WebhookURL,
+		bytes.NewReader(payload),
+	)
+	if err != nil {
+		return attemptResult{
+			success: false,
+			errMsg:  err.Error(),
+		}
+	}
+
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("User-Agent", "webhooker/1.0")
+
+	resp, doErr := executeHTTPRequest(t.client, req)
+	durationMs := time.Since(start).Milliseconds()
+
+	if doErr != nil {
+		return attemptResult{
+			success:  false,
+			duration: durationMs,
+			errMsg: fmt.Errorf(
+				"sending request: %w", doErr,
+			).Error(),
+		}
+	}
+
+	defer func() { _ = resp.Body.Close() }()
+
+	return t.readSlackResponse(resp, durationMs)
+}
+
+func (t *slackTarget) readSlackResponse(
+	resp *http.Response,
+	durationMs int64,
+) attemptResult {
+	body, readErr := io.ReadAll(
+		io.LimitReader(resp.Body, maxBodyLog),
+	)
+	if readErr != nil {
+		t.eng.log.Error(
+			"failed to read Slack response body",
+			"error", readErr,
+		)
+	}
+
+	success := resp.StatusCode >= httpSuccessMin &&
+		resp.StatusCode < httpSuccessMax
+
+	errMsg := ""
+	if !success {
+		errMsg = fmt.Sprintf("HTTP %d", resp.StatusCode)
+	}
+
+	return attemptResult{
+		statusCode: resp.StatusCode,
+		respBody:   string(body),
+		duration:   durationMs,
+		success:    success,
+		errMsg:     errMsg,
+	}
+}
+
+func parseSlackConfig(
+	configJSON string,
+) (*SlackTargetConfig, error) {
+	if configJSON == "" {
+		return nil, errEmptyTargetConfig
+	}
+
+	var cfg SlackTargetConfig
+
+	err := json.Unmarshal(
+		[]byte(configJSON), &cfg,
+	)
+	if err != nil {
+		return nil, fmt.Errorf(
+			"parsing config JSON: %w", err,
+		)
+	}
+
+	if cfg.WebhookURL == "" {
+		return nil, errMissingWebhookURL
+	}
+
+	return &cfg, nil
+}
+
+// FormatSlackMessage builds a Slack-compatible message
+// string from a webhook event.
+func FormatSlackMessage(
+	event *database.Event,
+) string {
+	var b strings.Builder
+
+	b.WriteString("*Webhook Event Received*\n")
+
+	fmt.Fprintf(
+		&b, "*Method:* `%s`\n", event.Method,
+	)
+
+	fmt.Fprintf(
+		&b,
+		"*Content-Type:* `%s`\n",
+		event.ContentType,
+	)
+
+	fmt.Fprintf(
+		&b,
+		"*Timestamp:* `%s`\n",
+		event.CreatedAt.UTC().Format(time.RFC3339),
+	)
+
+	fmt.Fprintf(
+		&b,
+		"*Body Size:* %d bytes\n",
+		len(event.Body),
+	)
+
+	if event.Body == "" {
+		b.WriteString("\n_(empty body)_\n")
+
+		return b.String()
+	}
+
+	if formatted := formatJSONBody(event.Body); formatted != "" {
+		b.WriteString(formatted)
+
+		return b.String()
+	}
+
+	formatRawBody(&b, event.Body)
+
+	return b.String()
+}
+
+func formatJSONBody(body string) string {
+	var parsed json.RawMessage
+	if json.Unmarshal([]byte(body), &parsed) != nil {
+		return ""
+	}
+
+	var pretty bytes.Buffer
+	if json.Indent(&pretty, parsed, "", "  ") != nil {
+		return ""
+	}
+
+	var b strings.Builder
+
+	b.WriteString("\n```\n")
+
+	prettyStr := pretty.String()
+
+	const maxPayloadDisplay = 3500
+	if len(prettyStr) > maxPayloadDisplay {
+		b.WriteString(prettyStr[:maxPayloadDisplay])
+		b.WriteString("\n... (truncated)")
+	} else {
+		b.WriteString(prettyStr)
+	}
+
+	b.WriteString("\n```\n")
+
+	return b.String()
+}
+
+func formatRawBody(b *strings.Builder, body string) {
+	b.WriteString("\n```\n")
+
+	const maxRawDisplay = 3500
+	if len(body) > maxRawDisplay {
+		b.WriteString(body[:maxRawDisplay])
+		b.WriteString("\n... (truncated)")
+	} else {
+		b.WriteString(body)
+	}
+
+	b.WriteString("\n```\n")
+}