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

The event log gains three links above the list: All, Failed (N) and
Pending (N), carried in a `show` query parameter; any other value shows
every event. Failed lists the events with a failed delivery, Pending
those with one pending or retrying, each once, under the full list's
50-row limit and order. A filtered list finds the matching deliveries
through `idx_deliveries_status`, looks up their events by ID and sorts
them, and reads the rows of only the events shown; its counts read the
deliveries alone. The line beside the heading says what a filter
counts, and Replay returns to the list it was pressed in. The README
and the comments naming the query parameters the service reads include
`show`.

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