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.

README.md

@@ -8,11 +8,11 @@ Written in Go using [Bubbletea](https://github.com/charmbracelet/bubbletea).
 
 - **Incident queue** — open incidents with severity, status, assignee and age, auto-refreshing
 - **Incident actions** — acknowledge, assign, snooze, note, resolve and archive
-- **Timeline** — the full history of an incident, system events and notes together
+- **Timeline** — the full history of an incident, system events, pages and notes together
 - **Alert feed** — the raw read-only alerts underneath, each linked to its incident
 - **On-call schedule** — visual calendar of who is on duty, assign and remove entries
 - **Statistics** — MTTA and MTTR, plus alert frequency by name, hour and day
-- **User management** — add and remove users, manage API keys
+- **User management** — add and remove users, manage API keys, set each user's ntfy topic
 
 > Requires terdut-server **v0.4.0 or later**. Earlier servers have no incidents API;
 > use terdut-tui v0.3.x with those.
@@ -37,6 +37,22 @@ Two behaviours worth knowing before you press a key:
 - **Snooze is the "not now" button.** It hides an incident from the default queue
   without closing it, and expires on its own.
 
+## Push notifications
+
+When the server is configured for ntfy, an incident that opens pages whoever is
+on call. Each user has their own topic, shown as a column in the Users section
+and edited with `t`. A user with no topic falls back to the server's shared
+fallback topic, which carries **no Acknowledge button** — the topic is shared, so
+a button on it would let any subscriber acknowledge as somebody else.
+
+Every delivery lands on the incident's timeline: `Notified <user> (triggered)`
+when ntfy accepted the page, and `Notification to <user> failed` when it ran out
+of retries. That second one is the one to look for when nobody's phone rang.
+
+Editing topics needs terdut-server **v0.6.0 or later**; the timeline entries need
+**v0.7.0 or later**. Against an older server the topic column stays empty and
+editing one reports the server's 404.
+
 ## Installation
 
 Download the latest release binary for your platform from the [releases page](https://github.com/yeniklas/terdut-tui/releases), or build from source:
@@ -128,5 +144,6 @@ Users section:
 | Key | Action |
 |-----|--------|
 | `n` | Create a user |
+| `t` | Edit the user's ntfy topic — submit empty to clear it |
 | `d` | Delete a user |
 | `k` | API keys for the selected user |