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/export_test.go

@@ -39,37 +39,50 @@ func ExportTruncate(s string, maxLen int) string {
 	return truncate(s, maxLen)
 }
 
-// ExportDeliverHTTP exposes deliverHTTP for testing.
+// ExportDeliverHTTP delivers via the http target for testing.
 func (e *Engine) ExportDeliverHTTP(
 	ctx context.Context,
 	webhookDB *gorm.DB,
 	d *database.Delivery,
 	task *Task,
 ) {
-	e.deliverHTTP(ctx, webhookDB, d, task)
+	e.httpTarget.Deliver(ctx, webhookDB, d, task, e)
 }
 
-// ExportDeliverDatabase exposes deliverDatabase.
+// ExportDeliverDatabase delivers via the database target.
 func (e *Engine) ExportDeliverDatabase(
 	webhookDB *gorm.DB, d *database.Delivery,
 ) {
-	e.deliverDatabase(webhookDB, d)
+	e.targets[database.TargetTypeDatabase].Deliver(
+		context.Background(), webhookDB, d, &Task{}, e,
+	)
 }
 
-// ExportDeliverLog exposes deliverLog for testing.
+// ExportDeliverLog delivers via the log target for testing.
 func (e *Engine) ExportDeliverLog(
 	webhookDB *gorm.DB, d *database.Delivery,
 ) {
-	e.deliverLog(webhookDB, d)
+	e.targets[database.TargetTypeLog].Deliver(
+		context.Background(), webhookDB, d, &Task{}, e,
+	)
 }
 
-// ExportDeliverSlack exposes deliverSlack for testing.
+// ExportDeliverSlack delivers via the slack target for
+// testing.
 func (e *Engine) ExportDeliverSlack(
 	ctx context.Context,
 	webhookDB *gorm.DB,
 	d *database.Delivery,
 ) {
-	e.deliverSlack(ctx, webhookDB, d)
+	task := &Task{
+		DeliveryID: d.ID,
+		TargetID:   d.TargetID,
+		AttemptNum: 1,
+	}
+
+	e.targets[database.TargetTypeSlack].Deliver(
+		ctx, webhookDB, d, task, e,
+	)
 }
 
 // ExportProcessNewTask exposes processNewTask.
@@ -96,53 +109,56 @@ func (e *Engine) ExportProcessDelivery(
 	e.processDelivery(ctx, webhookDB, d, task)
 }
 
-// ExportGetCircuitBreaker exposes getCircuitBreaker.
+// ExportGetCircuitBreaker exposes the http target's
+// getCircuitBreaker.
 func (e *Engine) ExportGetCircuitBreaker(
 	targetID string,
 ) *CircuitBreaker {
-	return e.getCircuitBreaker(targetID)
+	return e.httpTarget.getCircuitBreaker(targetID)
 }
 
 // ExportParseHTTPConfig exposes parseHTTPConfig.
 func (e *Engine) ExportParseHTTPConfig(
 	configJSON string,
 ) (*HTTPTargetConfig, error) {
-	return e.parseHTTPConfig(configJSON)
+	return parseHTTPConfig(configJSON)
 }
 
 // ExportParseSlackConfig exposes parseSlackConfig.
 func (e *Engine) ExportParseSlackConfig(
 	configJSON string,
 ) (*SlackTargetConfig, error) {
-	return e.parseSlackConfig(configJSON)
+	return parseSlackConfig(configJSON)
 }
 
-// ExportDoHTTPRequest exposes doHTTPRequest.
+// ExportDoHTTPRequest exposes the http target's
+// doHTTPRequest.
 func (e *Engine) ExportDoHTTPRequest(
 	ctx context.Context,
 	cfg *HTTPTargetConfig,
 	event *database.Event,
 ) (int, string, int64, error) {
-	return e.doHTTPRequest(ctx, cfg, event)
+	return e.httpTarget.doHTTPRequest(ctx, cfg, event)
 }
 
-// ExportClientForConfig exposes clientForConfig.
+// ExportClientForConfig exposes the http target's
+// clientForConfig.
 func (e *Engine) ExportClientForConfig(
 	cfg *HTTPTargetConfig,
 ) *http.Client {
-	return e.clientForConfig(cfg)
+	return e.httpTarget.clientForConfig(cfg)
 }
 
-// ExportClient returns the engine's shared HTTP client.
+// ExportClient returns the http target's shared HTTP client.
 func (e *Engine) ExportClient() *http.Client {
-	return e.client
+	return e.httpTarget.client
 }
 
-// ExportScheduleRetry exposes scheduleRetry.
+// ExportScheduleRetry exposes ScheduleRetry.
 func (e *Engine) ExportScheduleRetry(
 	task Task, delay time.Duration,
 ) {
-	e.scheduleRetry(task, delay)
+	e.ScheduleRetry(task, delay)
 }
 
 // ExportRecoverPendingDeliveries exposes
@@ -199,13 +215,15 @@ func NewTestEngine(
 	client *http.Client,
 	workers int,
 ) *Engine {
-	return &Engine{
+	e := &Engine{
 		log:        log,
-		client:     client,
 		deliveryCh: make(chan Task, deliveryChannelSize),
 		retryCh:    make(chan Task, retryChannelSize),
 		workers:    workers,
 	}
+	e.initTargets(client)
+
+	return e
 }
 
 // NewTestEngineSmallRetry creates an Engine with a tiny
@@ -213,10 +231,13 @@ func NewTestEngine(
 func NewTestEngineSmallRetry(
 	log *slog.Logger,
 ) *Engine {
-	return &Engine{
+	e := &Engine{
 		log:     log,
 		retryCh: make(chan Task, 1),
 	}
+	e.initTargets(nil)
+
+	return e
 }
 
 // NewTestEngineWithDB creates an Engine with a real
@@ -228,15 +249,17 @@ func NewTestEngineWithDB(
 	client *http.Client,
 	workers int,
 ) *Engine {
-	return &Engine{
+	e := &Engine{
 		database:   db,
 		dbManager:  dbMgr,
 		log:        log,
-		client:     client,
 		deliveryCh: make(chan Task, deliveryChannelSize),
 		retryCh:    make(chan Task, retryChannelSize),
 		workers:    workers,
 	}
+	e.initTargets(client)
+
+	return e
 }
 
 // NewTestCircuitBreaker creates a CircuitBreaker with