Move webhook pages to /hook/ID and inbound URLs to /h/UUID (closes #367)

The webhook page and everything under it move from /source/ID to
/hook/ID, the list and new-webhook form to /hooks and /hooks/new, and
the event log from .../logs to /hook/ID/events, body download
included. Entrypoint URLs move from /webhook/UUID to /h/UUID, and the
webhook page shows only that form. The old paths are gone.

Links, redirects, form actions, tests, comments and the README follow.
Go identifiers and template file names are unchanged. A new route test
posts to the entrypoint URL the webhook page shows and checks that the
receiver rate limit applies to it.

Model: opus-5-5

internal/middleware/csrf.go

@@ -41,7 +41,7 @@ func (m *Middleware) CSRF() func(http.Handler) http.Handler {
 		// CSRF is registered ahead of RequireAuth on every route
 		// group that uses it, so this WARN is reachable by an
 		// unauthenticated client: a POST with no token to
-		// /source/<any length of any text>/edit lands here. The
+		// /hook/<any length of any text>/edit lands here. The
 		// method and path are capped against the same budgets as
 		// the access log. remote_addr is set by net/http from the
 		// accepted connection rather than by the client, and