Show each webhook's activity in the webhook list (closes #394)

Each entry in the webhook list at /hooks now shows when its last event arrived (or "No events yet"), how many of its deliveries failed in the last 24 hours, in red when not zero, and how many entrypoints and targets are inactive, as in "4 targets, 1 inactive". The figures come from the event totals row and the statistics pane's own query, so the list no longer counts every stored event and its event count matches the pane's. A webhook whose event database cannot be read says so in its entry; the rest of the list still shows. A failed read of entrypoints or targets from the main database now returns an error page instead of showing zero.

Model: opus-5-5

README.md

@@ -1786,7 +1786,7 @@ retries) is individually logged for full observability.
 #### EventTotals and TargetTotals
 
 Running counts in each event database, read by the statistics pane at the
-top of the webhook page. `EventTotals` is one row:
+top of the webhook page and by the webhook list. `EventTotals` is one row:
 
 | Field            | Type      | Description |
 | ---------------- | --------- | ----------- |
@@ -1818,6 +1818,14 @@ target. Its failure percentage for a window is the deliveries that became
 `failed` in it out of all that became `delivered` or `failed` in it, and
 a dash when none did.
 
+The webhook list at `/hooks` shows three of the pane's figures for each
+webhook: its events within retention and its last event, both from
+`EventTotals`, and its deliveries that failed in the last 24 hours,
+counted with the pane's query. It opens each webhook's event database once
+(the handle stays open) and runs those two reads there, so its cost grows
+with the number of webhooks and, for each, with the deliveries that
+finished in the last 24 hours, never with the events stored.
+
 #### Event-tier indexes
 
 These indexes on the per-webhook event databases are declared in the model
@@ -1825,7 +1833,7 @@ 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, 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 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 |