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/api/client_test.go

@@ -83,6 +83,8 @@ func TestClient_IncidentEndpoints(t *testing.T) {
 			http.MethodDelete, "/api/incidents/7/notes/12", ""},
 		{"stats", func(c *Client) error { _, err := c.GetIncidentStats(); return err },
 			http.MethodGet, "/api/stats/incidents", ""},
+		{"set notify target", func(c *Client) error { _, err := c.SetUserNotifyTarget(7, "t"); return err },
+			http.MethodPut, "/api/users/7/notify", ""},
 	}
 
 	for _, tt := range tests {
@@ -163,6 +165,43 @@ func TestClient_RequestBodies(t *testing.T) {
 			t.Errorf("expected duration 90m, got %q", body.Duration)
 		}
 	})
+
+	t.Run("set notify target", func(t *testing.T) {
+		c, got := stub(t, http.StatusOK, `{}`)
+		if _, err := c.SetUserNotifyTarget(3, "terdut-niklas"); err != nil {
+			t.Fatalf("set notify target: %v", err)
+		}
+		if got.body != `{"ntfy_topic":"terdut-niklas"}` {
+			t.Errorf("unexpected body %q", got.body)
+		}
+	})
+
+	// Clearing has to put an explicit empty string on the wire: omitting the
+	// field would leave the topic untouched instead of removing it.
+	t.Run("clear notify target", func(t *testing.T) {
+		c, got := stub(t, http.StatusOK, `{}`)
+		if _, err := c.SetUserNotifyTarget(3, ""); err != nil {
+			t.Fatalf("clear notify target: %v", err)
+		}
+		if got.body != `{"ntfy_topic":""}` {
+			t.Errorf("expected an explicit empty topic, got %q", got.body)
+		}
+	})
+}
+
+func TestUser_TopicFlattensNilAndEmpty(t *testing.T) {
+	var users []User
+	if err := json.Unmarshal([]byte(
+		`[{"id":1,"username":"a"},{"id":2,"username":"b","ntfy_topic":""},
+		  {"id":3,"username":"c","ntfy_topic":"terdut-c"}]`), &users); err != nil {
+		t.Fatalf("decode: %v", err)
+	}
+	want := []string{"", "", "terdut-c"}
+	for i, u := range users {
+		if got := u.Topic(); got != want[i] {
+			t.Errorf("user %d: expected topic %q, got %q", u.ID, want[i], got)
+		}
+	}
 }
 
 // The 409 on re-resolving is the server telling the user why nothing happened,