Show an event's entrypoint and request headers (closes #389)

An expanded event in the event log and the event's own page now show
the entrypoint the event arrived at (its description, "Entrypoint"
when it has none, or "deleted entrypoint"; never its URL), or for a
resubmitted copy the one its original arrived at, and the request
headers, one per line, sorted by name, whitespace kept. The event log
leaves out headers that hold more than the body's 32 KiB limit, stored
or as lines, and links to the event's page, which shows them all.
Both pages draw them through one shared template, event_request.html,
and read the webhook's entrypoints once per page.

Model: opus-5-5

README.md

@@ -3104,7 +3104,7 @@ returns to the page that was asked for.
 | `POST` | `/hook/{id}/edit`        | Edit webhook submission |
 | `POST` | `/hook/{id}/delete`      | Delete webhook |
 | `GET`  | `/hook/{id}/events`      | Full Event Log |
-| `GET`  | `/hook/{id}/events/{eventID}` | One event's own page: its details, its whole body and every delivery of it |
+| `GET`  | `/hook/{id}/events/{eventID}` | One event's own page: its details, the entrypoint it arrived at (for a resubmitted copy, the one its original arrived at), its request headers, its whole body and every delivery of it |
 | `GET`  | `/hook/{id}/events/{eventID}/body` | Download an event's stored body. The pages show a body as text, cut at 32 KiB in the recent events and the event log, and leave a binary one out, so this is the only route that serves the stored bytes; it is offered wherever a body is cut or binary |
 | `POST` | `/hook/{id}/deliveries/{deliveryID}/replay` | Replay a finished delivery: creates a new delivery for the same event against the target's current configuration (30 per minute per bucket, then `429`) |
 | `POST` | `/hook/{id}/events/{eventID}/resubmit` | Resubmit a stored event: creates a new event copying it and fans that out to every currently active target (30 per minute per bucket, then `429`) |