Show each target's delivered and failed deliveries in the target list (closes #372)

Each row of the webhook page's target list now shows the target's
delivered and failed deliveries, in total and in the last 24 hours.
The totals are the target's running totals, so retention does not
reduce them; the 24-hour figures are counted over that window of the
deliveries' status index, in one query for all targets. Deliveries
still pending or retrying count in neither.

Model: opus-5-5

README.md

@@ -1896,6 +1896,12 @@ counted with the pane's query. It opens each webhook's event database once
 with the number of webhooks and, for each, with the deliveries that
 finished in the last 24 hours, never with the events stored.
 
+The target list on the webhook page shows, for each target, its
+`delivered` and `failed` totals, which retention does not reduce, and its
+deliveries that became `delivered` and `failed` in the last 24 hours,
+counted with the pane's query. Deliveries still `pending` or `retrying`
+count in neither.
+
 #### Event-tier indexes
 
 These indexes on the per-webhook event databases are declared in the model
@@ -1903,7 +1909,7 @@ tags, so `AutoMigrate` creates them on a fresh database:
 
 | Table              | Columns                     | Serves |
 | ------------------ | --------------------------- | ------ |
-| `deliveries`       | `status`, `deleted_at`, `finished_at`, `target_id` | Startup recovery, the retry and pending sweeps every 60 seconds and the queue-depth sampler every 30 seconds, which select deliveries by status, and the webhook page's statistics and the webhook list, which count each target's deliveries by status and when they finished |
+| `deliveries`       | `status`, `deleted_at`, `finished_at`, `target_id` | Startup recovery, the retry and pending sweeps every 60 seconds and the queue-depth sampler every 30 seconds, which select deliveries by status, and the webhook page's statistics and target list and the webhook list, which count each target's deliveries by status and when they finished |
 | `deliveries`       | `event_id`, `deleted_at`    | The event log, which loads each event's deliveries, and retention, which counts and deletes the deliveries of expired events |
 | `delivery_results` | `delivery_id`, `deleted_at` | The event log, which loads the attempts of a page's deliveries, and retention, which deletes the attempts of expired events |
 | `events`           | `deleted_at`, `created_at`  | The webhook page's statistics, which count recent events |