Event log: show only the events with a failed or a pending delivery (closes #390)

The event log had no way to list only the events whose delivery failed, and once it showed only the 50 newest, an older failure could not be found at all. It now has All, Failed (N) and Pending (N) links, carried in a `show` query parameter, so they work without the page's script library. Each filtered list keeps the 50-row limit and newest-first order, and lists an event once. It finds matching deliveries through `idx_deliveries_status` and looks their events up by ID, so its cost follows the matches, not the webhook's size. Replay returns to the list it was pressed in. The heading line says what a filter counts.

Model: opus-5-5

internal/middleware/middleware.go

@@ -279,8 +279,9 @@ func (lrw *loggingResponseWriter) Unwrap() http.ResponseWriter {
 // this branch as bounded as the pattern branches below.
 //
 // Nothing debuggable is lost. The only query parameters the service
-// reads are the login page's `next`, the page to return to, and
-// `notice`, which names the line a page shows after an action. The
+// reads are the login page's `next`, the page to return to,
+// `notice`, which names the line a page shows after an action, and
+// the event log's `show`, which picks the events it lists. The
 // alternatives that would preserve more (a key count, a key
 // allowlist) all require parsing an attacker-sized query on every
 // request, which is work an unauthenticated client would then be