Event log: show attempt and delivery times, label replays, zone event times (closes #386)

Each attempt shows when it was recorded and each delivery when it was
created, in the event log and on the event's page. The event log's
event times read as the recent events list does: how long ago, with
the full UTC time on hover.

A delivery records whether Replay created it, in a new replay column
added to the delivery model in place. Such a delivery is labelled a
replay in the event's summary line and in both pages' delivery lists.

Model: opus-5-5

README.md

@@ -1830,6 +1830,7 @@ status across potentially multiple attempts.
 | `target_id`| UUID           | Foreign key → Target |
 | `status`   | DeliveryStatus | One of: `pending`, `delivered`, `failed`, `retrying` |
 | `finished_at` | timestamp   | When the delivery became `delivered` or `failed` (nullable; empty while `pending` or `retrying`) |
+| `replay`   | boolean        | Whether the delivery was created by **Replay** |
 
 **Relations:** Belongs to Event. Belongs to Target. Has many
 DeliveryResults.
@@ -1849,7 +1850,8 @@ NEW `pending` delivery for the same event and target and hands it to
 the engine on the ordinary path — same retries, same SSRF guard, same
 circuit breaker as a first attempt. It never touches the delivery it
 repeats: that row's status, timestamps and recorded attempts stand as
-the record of what happened.
+the record of what happened. The new delivery records `replay`, and the
+event log and the event's page label it a replay.
 
 What is re-sent is the stored event body, against the target's
 configuration **as it stands now** — the point of a replay is to
@@ -1903,6 +1905,10 @@ A `database` or `log` target sends no HTTP request, so in the event log and
 on the event's page its attempts show no status: a successful one reads
 "archived" or "written to the log".
 
+The event log and the event's page show when each attempt was recorded and
+when each delivery was created, as the recent events list shows when an event
+arrived: how long ago, with the full UTC time on hover.
+
 **Relations:** Belongs to Delivery.
 
 #### EventTotals, TargetTotals and EntrypointTotals