Refactor delivery targets to a Target interface (closes #77) (#81)

Refactors the delivery engine so each target TYPE is an implementation of a `Target` interface, dispatched from a registry, with each target owning its full delivery including durable retries. Implements the authoritative design from issue #77 (the corrected "hand the DB + Scheduler to the target" design).

## The new interface

```go
type Scheduler interface {
    ScheduleRetry(task Task, delay time.Duration)
}

type Target interface {
    Deliver(ctx context.Context, webhookDB *gorm.DB,
        d *database.Delivery, task *Task, sched Scheduler)
}
```

`Deliver` receives everything a target needs to be autonomous and durable: the request context, the per-webhook `*gorm.DB`, the `*database.Delivery`, the attempt `*Task`, and a `Scheduler` (the engine) for durable re-enqueue. The target makes one attempt, writes 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 injected `Scheduler`.

`processDelivery` collapses to a registry lookup (`map[database.TargetType]Target`) and a `Deliver` call; an unknown target type still fails the delivery as before.

## Per-target ownership

- `httpTarget` and `slackTarget` share a retry core (`httpCore`) that owns retry, exponential backoff, and the per-target circuit breaker. The core is fire-and-forget when `MaxRetries == 0` and adds breaker-gated backed-off retries when `MaxRetries > 0`. The per-attempt request differs (HTTP forwards the body + filtered headers; Slack posts a formatted message) and is supplied as a closure, so each keeps its exact recording semantics (e.g. HTTP records no error string for a non-2xx, Slack records `HTTP <code>`).
- `databaseTarget` and `logTarget` are fire-and-forget: they record a single successful attempt.

Moved wholesale into the http/slack targets: `deliverHTTP*`, `handleHTTPRetry`, `circuitBreakerBlock`, `calcBackoff` / `calcRemainingBackoff` / `backoffElapsed`, the circuit-breaker `sync.Map` + `getCircuitBreaker`, `clientForConfig`, `doHTTPRequest`, `applyRequestHeaders`, and the config parsers. The engine keeps `recordResult`, `updateDeliveryStatus`, and `ScheduleRetry`.

## Slack MaxRetries gating

Slack is now on the same shared core as HTTP, with retry + breaker gated on `MaxRetries`. A `MaxRetries` of 0 stays single-attempt fire-and-forget, so **every existing Slack target is unchanged**; a Slack target configured with retries gets backoff + circuit breaker.

## Log-target full content

`logTarget` now logs the ENTIRE inbound webhook — full request body and full request headers, plus method, content type, and the webhook id and entrypoint id — rather than a summary line. This supersedes the smaller log-summary work (#70).

## `Task.EntrypointID`

To carry the entrypoint id to the log target, `Task` gains an `EntrypointID` field, populated in the webhook handler's `buildDeliveryTasks`, the engine's recovery-task builder, and `buildEventFromTask`.

## Durability / recovery

The crash-durable async retry model is preserved unchanged: one attempt per worker turn; on failure the status is set `retrying`, backoff is computed, and the task is re-enqueued via `ScheduleRetry` (a `time.AfterFunc` onto the retry channel). On restart, `recoverRetryingDeliveries` and the 60s sweep hand each orphaned `retrying` delivery back to its target to recompute the remaining backoff and reschedule (targets that own retries implement an internal `rescheduler`; fire-and-forget targets, which never produce `retrying` deliveries, are skipped).

## How behaviour is preserved

No external behaviour changes except the two called out above (log target full content; Slack gaining `MaxRetries`-gated retries). All existing delivery tests pass with only their `export_test.go` wrappers re-pointed at the new structure — `ExportDeliverHTTP/Slack/Database/Log` now call the targets, `ExportGetCircuitBreaker` / `ExportClient` / `ExportClientForConfig` / `ExportDoHTTPRequest` resolve against the HTTP target's shared client and breaker map, and `ExportParseHTTPConfig` / `ExportParseSlackConfig` call the relocated free functions. Added: a `logTarget` test asserting the log line contains the full body, headers, and ids, and a Slack `MaxRetries`-gated retry test.

`docker build .` is green (fmt-check, lint, test, static build all pass).

Closes #77

Co-authored-by: sneak <sneak@sneak.berlin>
Reviewed-on: #81
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/delivery/engine_test.go

@@ -1,6 +1,7 @@
 package delivery_test
 
 import (
+	"bytes"
 	"context"
 	"database/sql"
 	"encoding/json"
@@ -1652,6 +1653,179 @@ func TestProcessDelivery_RoutesToSlack(t *testing.T) {
 	)
 }
 
+// newLogCaptureEngine builds a test engine whose logger
+// writes to the returned buffer, for inspecting log output.
+func newLogCaptureEngine(
+	t *testing.T,
+) (*delivery.Engine, *bytes.Buffer) {
+	t.Helper()
+
+	var buf bytes.Buffer
+
+	log := slog.New(slog.NewTextHandler(
+		&buf,
+		&slog.HandlerOptions{Level: slog.LevelDebug},
+	))
+
+	e := delivery.NewTestEngine(
+		log, &http.Client{Timeout: 5 * time.Second}, 1,
+	)
+
+	return e, &buf
+}
+
+// assertLogLineComplete asserts the captured log output
+// carries the full inbound webhook content and ids.
+func assertLogLineComplete(
+	t *testing.T, out string, event database.Event,
+) {
+	t.Helper()
+
+	assert.Contains(t, out, "log-body-marker",
+		"log line must contain the full request body",
+	)
+
+	assert.Contains(t, out, "Content-Type",
+		"log line must contain the full request headers",
+	)
+
+	assert.Contains(t, out, event.EntrypointID,
+		"log line must contain the entrypoint id",
+	)
+
+	assert.Contains(t, out, event.WebhookID,
+		"log line must contain the webhook id",
+	)
+
+	assert.Contains(t, out, "application/json",
+		"log line must contain the content type",
+	)
+}
+
+func TestDeliverLog_LogsFullContent(t *testing.T) {
+	t.Parallel()
+
+	db := testWebhookDB(t)
+	e, buf := newLogCaptureEngine(t)
+
+	event := seedEvent(
+		t, db, `{"log-body-marker":"abc123"}`,
+	)
+
+	dlv := seedDelivery(
+		t, db, event.ID, uuid.New().String(),
+		database.DeliveryStatusPending,
+	)
+
+	d := &database.Delivery{
+		EventID:  event.ID,
+		TargetID: dlv.TargetID,
+		Status:   database.DeliveryStatusPending,
+		Event:    event,
+		Target: database.Target{
+			Name: "test-log-full",
+			Type: database.TargetTypeLog,
+		},
+	}
+	d.ID = dlv.ID
+
+	e.ExportDeliverLog(db, d)
+
+	assertLogLineComplete(t, buf.String(), event)
+
+	assertDeliveryStatus(t, db, dlv.ID,
+		database.DeliveryStatusDelivered,
+	)
+}
+
+// buildSlackRetryDelivery builds a Slack delivery whose
+// target is configured with retries enabled.
+func buildSlackRetryDelivery(
+	dlv database.Delivery,
+	event database.Event,
+	targetID, cfg string,
+) *database.Delivery {
+	d := &database.Delivery{
+		EventID:  event.ID,
+		TargetID: targetID,
+		Status:   database.DeliveryStatusPending,
+		Event:    event,
+		Target: database.Target{
+			Name:       "test-slack-retry",
+			Type:       database.TargetTypeSlack,
+			Config:     cfg,
+			MaxRetries: 5,
+		},
+	}
+	d.ID = dlv.ID
+
+	return d
+}
+
+func TestDeliverSlack_WithRetries_SchedulesRetry(
+	t *testing.T,
+) {
+	t.Parallel()
+
+	db := testWebhookDB(t)
+	ts := newStatusServer(t, http.StatusServiceUnavailable)
+	e := testEngine(t, 1)
+	targetID := uuid.New().String()
+
+	slackCfg, err := json.Marshal(
+		delivery.SlackTargetConfig{WebhookURL: ts.URL},
+	)
+	require.NoError(t, err)
+
+	event := seedEvent(t, db, `{"slack":"retry"}`)
+
+	dlv := seedDelivery(
+		t, db, event.ID, targetID,
+		database.DeliveryStatusPending,
+	)
+
+	d := buildSlackRetryDelivery(
+		dlv, event, targetID, string(slackCfg),
+	)
+
+	task := &delivery.Task{
+		DeliveryID: dlv.ID,
+		TargetID:   targetID,
+		TargetType: database.TargetTypeSlack,
+		MaxRetries: 5,
+		AttemptNum: 1,
+	}
+
+	e.ExportProcessDelivery(context.TODO(), db, d, task)
+
+	assertDeliveryStatus(t, db, dlv.ID,
+		database.DeliveryStatusRetrying,
+	)
+
+	assertDeliveryResult(
+		t, db, dlv.ID, false,
+		http.StatusServiceUnavailable,
+	)
+}
+
+// newStatusServer starts a test server that always responds
+// with the given status code.
+func newStatusServer(
+	t *testing.T, code int,
+) *httptest.Server {
+	t.Helper()
+
+	ts := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, _ *http.Request) {
+			w.WriteHeader(code)
+		},
+	))
+
+	t.Cleanup(ts.Close)
+
+	return ts
+}
+
 // readAll is a small helper to avoid importing io in
 // a test handler inline.
 func readAll(r interface {