Add per-delivery replay to the event log (closes #203) (#240)

There was no redelivery path anywhere: once a delivery exhausted
max_retries it was failed permanently, even though the event body is
durably stored. Storing an event and being unable to re-send it defeats
the reason it is stored, and the ordinary case is a destination that was
down longer than the backoff ladder.

Adds POST /source/{sourceID}/deliveries/{deliveryID}/replay, inside the
authenticated group so it inherits MaxBodySize, CSRF, NoCache and
RequireAuth. Replay creates a NEW pending delivery against the target's
CURRENT config and hands it to the engine through the same notifier the
receiver uses, so it runs the normal path with the retry ladder, the
SSRF-guarded transport and the circuit breaker. The original delivery's
rows are never touched, and the stored event body is re-sent, never the
recorded response.

Replay is refused, with a distinct message, for a non-terminal delivery, a
deleted target, a deactivated target, and when an earlier replay of the
same event and target is still in flight. Bounded by a per-client rate
limit and by that in-flight check.

The new delivery row is written with Omit(clause.Associations) and with
neither Event nor Target populated, so it cannot upsert a targets row into
the per-webhook event database (#206).

Counted by webhooker_delivery_replays_total on the existing target_type
label. A replay also moves the ordinary attempt, outcome and duration
series, because it is a real delivery.

README.md

@@ -1108,6 +1108,23 @@ DeliveryResults.
   succeeded).
 - **`failed`** — All retry attempts exhausted without success.
 
+**Replay.** A `delivered` or `failed` delivery is finished as far as
+the engine is concerned, but the event is still stored, so the event
+log offers a per-delivery **Replay** action for it. Replay creates a
+NEW `pending` delivery for the same event and target and hands it to
+the engine on the ordinary path — same retries, same SSRF guard, same
+circuit breaker as a first attempt. It never touches the delivery it
+repeats: that row's status, timestamps and recorded attempts stand as
+the record of what happened.
+
+What is re-sent is the stored event body, against the target's
+configuration **as it stands now** — the point of a replay is to
+deliver where the destination has since been fixed. A target that has
+been deleted or deactivated therefore refuses the replay with a
+message on the event log rather than delivering from stale
+configuration, and a replay is refused while an earlier one for the
+same event and target is still pending or retrying.
+
 #### DeliveryResult
 
 The result of a single delivery attempt. Every attempt (including
@@ -1439,6 +1456,7 @@ arriving and being stored, they are just not getting anywhere.
 | `webhooker_deliveries_succeeded_total` | counter | Deliveries that reached `delivered` |
 | `webhooker_deliveries_failed_total` | counter | Deliveries that failed terminally and will not be retried |
 | `webhooker_delivery_retries_total` | counter | Deliveries put back into `retrying` |
+| `webhooker_delivery_replays_total` | counter | Deliveries an operator replayed from the event log. A replay runs the ordinary engine path, so it also moves the attempt, outcome and duration series; this is the only one that separates it from ordinary traffic |
 | `webhooker_delivery_duration_seconds` | histogram | Wall time of a single dispatched delivery attempt, the same duration the attempt's `DeliveryResult` records |
 | `webhooker_deliveries_pending` | gauge | Deliveries currently in `pending` |
 | `webhooker_deliveries_retrying` | gauge | Deliveries currently in `retrying` |
@@ -2036,6 +2054,7 @@ abuse limit later; they are tracked as future work.
 | `POST` | `/source/{id}/edit`      | Edit webhook submission |
 | `POST` | `/source/{id}/delete`    | Delete webhook |
 | `GET`  | `/source/{id}/logs`      | Webhook event logs |
+| `POST` | `/source/{id}/deliveries/{deliveryID}/replay` | Replay a finished delivery: creates a new delivery for the same event against the target's current configuration (30 per minute per bucket, then `429`) |
 | `POST` | `/source/{id}/entrypoints` | Add entrypoint to webhook |
 | `POST` | `/source/{id}/entrypoints/{entrypointID}/delete` | Delete an entrypoint |
 | `POST` | `/source/{id}/entrypoints/{entrypointID}/toggle` | Enable or disable an entrypoint |
@@ -2125,6 +2144,7 @@ webhooker/
 │   ├── handlers/
 │   │   ├── handlers.go        # Base handler struct, JSON helpers, template rendering
 │   │   ├── auth.go            # Login, logout handlers
+│   │   ├── delivery_replay.go # Per-delivery replay: new delivery, current target config
 │   │   ├── entrypoint_view.go # Masked entrypoint view for templates
 │   │   ├── event_log_view.go  # Event log projection, byte-capped in SQL
 │   │   ├── healthcheck.go     # Health check handler