feat: ntfy topics per user, and notifications on the timeline

terdut-server pages the on-call person through ntfy, but none of it was
reachable from here. A user's topic could only be set with curl, so a
new user silently got no pages and quietly fell back to the shared
fallback topic — which carries no Acknowledge button. And nothing said
whether anybody had been paged at all.

The Users section grows an Ntfy Topic column and t to edit it,
prefilled with the current value. Submitting an empty field clears the
topic rather than being rejected as a mistake: clearing is how somebody
is taken off their own topic, and it is what the server means by an
empty string. Nil and empty arrive as the same thing, because the
server stores a blank topic as NULL, so User.Topic flattens the two
instead of leaving every caller to.

The incident timeline renders the server's notified and notify_failed
events. No new fetch — the timeline endpoint already carried them, and
unknown types already fell through to a generic label; this is about
saying something useful. An event with no user means the fallback
topic, not "the server acted", which is the difference between somebody
having been paged and the rota having been empty.

Both need terdut-server v0.6.0 or later, and the timeline entries a
server newer than that. Against an older one the column stays empty and
editing a topic reports the server's 404, which is the honest answer.

internal/tui/view.go

@@ -73,6 +73,8 @@ func (m Model) renderBody() string {
 		return m.renderUserPicker()
 	case modeUserCreate:
 		return m.renderUserCreate()
+	case modeUserNotifyEdit:
+		return m.renderUserNotifyEdit()
 	case modeAPIKeyMenu:
 		return m.renderAPIKeyMenu()
 	case modeAPIKeyCreate:
@@ -127,6 +129,9 @@ func (m Model) renderFooter() string {
 	case modeUserCreate:
 		return withStatus("  tab·next field  enter·create  esc·cancel")
 
+	case modeUserNotifyEdit:
+		return withStatus("  enter·save  esc·cancel   (empty clears the topic)")
+
 	case modeAPIKeyMenu:
 		return withStatus("  n·new key  r·revoke by ID  esc·back")
 
@@ -152,7 +157,7 @@ func (m Model) renderFooter() string {
 		case sectionSchedule:
 			return withStatus("  +·assign day  W·assign week  d·del  ←/→·shift week  tab·section  r·refresh  q·quit")
 		case sectionUsers:
-			return withStatus("  n·new user  d·delete  k·API keys  r·refresh  tab·section  q·quit")
+			return withStatus("  n·new user  t·topic  d·delete  k·API keys  r·refresh  tab·section  q·quit")
 		}
 		return "\n" + styleFooter.Render(m.help.ShortHelpView(m.keys.ShortHelp()))
 	}
@@ -550,6 +555,15 @@ func eventLabel(e api.IncidentEvent) string {
 			return "  Resolved by " + who
 		}
 		return "  Resolved (all alerts stopped firing)"
+	case api.EventNotified:
+		// An empty username here is not "the server acted": it means the page
+		// went to the shared fallback topic, so it belongs to nobody.
+		return fmt.Sprintf("  Notified %s%s", notifiedTarget(who), notifyKind(e.Detail))
+	case api.EventNotifyFailed:
+		// The detail is "<kind>: <reason>", and the reason is the point — it is
+		// the only thing that says why nobody's phone rang.
+		return truncate(fmt.Sprintf("  Notification to %s failed · %s",
+			notifiedTarget(who), e.Detail), 52)
 	default:
 		label := "  " + e.Type
 		if e.Detail != "" {
@@ -559,6 +573,25 @@ func eventLabel(e api.IncidentEvent) string {
 	}
 }
 
+// notifiedTarget names who a page reached. The server attaches no user when it
+// published to the shared fallback topic, and saying so is the difference
+// between "somebody was paged" and "the on-call rota was empty".
+func notifiedTarget(username string) string {
+	if username == "" {
+		return "the fallback topic"
+	}
+	return username
+}
+
+// notifyKind renders the notification kind the server puts in Detail. It is an
+// open set, so anything unrecognised is shown rather than dropped.
+func notifyKind(detail string) string {
+	if detail == "" {
+		return ""
+	}
+	return " (" + detail + ")"
+}
+
 func buildAlertDetailContent(alert api.Alert, width int) string {
 	now := time.Now()
 	var b strings.Builder
@@ -733,6 +766,16 @@ func (m Model) renderUserCreate() string {
 		emailLabel + m.userFormInputs[1].View() + "\n"
 }
 
+func (m Model) renderUserNotifyEdit() string {
+	header := fmt.Sprintf("\n  Push notifications for %s\n", styleBold.Render(m.selectedUser.Username))
+	hint := line(styleMuted,
+		"  The ntfy topic this user's pages go to. Leave it empty to clear it —\n"+
+			"  their incidents then page the server's shared fallback topic, which\n"+
+			"  carries no Acknowledge button.")
+	label := styleSelected.Render("  Topic:  ")
+	return header + "\n" + hint + "\n" + label + m.ntfyTopicInput.View() + "\n"
+}
+
 func (m Model) renderAPIKeyMenu() string {
 	header := fmt.Sprintf("\n  API keys for %s\n", styleBold.Render(m.selectedUser.Username))
 	warning := line(styleMuted, "  Keys cannot be listed — only new keys can be created,\n  or existing ones revoked by their integer ID.")