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

The target list showed nothing about how a target's deliveries were going. Each target now shows Delivered and Failed, each in total and in the last 24 hours. The totals are the per-target running totals kept for the statistics pane, so retention does not reduce them; the 24-hour figures are one count over the deliveries' final-status index, for all of the webhook's targets at once. Pending and retrying deliveries count in neither. If the event database cannot be read, each row says so instead of showing zeros. The archive details and Download button on database targets are kept.

Model: opus-5-5

README.md

@@ -1913,6 +1913,12 @@ 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
@@ -1920,7 +1926,7 @@ 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 |