docs: the ack token is scoped, not single-use

The handler never deletes the token: it stays valid until expires_at and
is purged by the sweeper, so a second tap is an idempotent no-op rather
than a rejection. Caught by pressing Acknowledge twice against the live
server. What bounds the token is scope -- one incident, one action, one
day -- not a use count.

README.md

@@ -211,10 +211,17 @@ Three things get pushed:
 
 Notifications carry an **Acknowledge** button that acknowledges the incident
 without opening anything. It POSTs to `/api/notify/ack/{token}`, an
-unauthenticated route authorised by the 256-bit single-use token in its path —
-minted fresh per notification, scoped to one incident and one action, and valid
-for 24 hours. A real API key is never put in a notification, because the message
-is stored on the ntfy server and cached on the device.
+unauthenticated route authorised by the 256-bit token in its path — minted fresh
+per notification, scoped to one incident and one action, and valid for 24 hours.
+A real API key is never put in a notification, because the message is stored on
+the ntfy server and cached on the device.
+
+The token is **not** consumed by use. Acknowledging is idempotent, so a token
+stays valid for its full 24 hours and a second tap is a no-op that reports the
+incident's current state rather than an error — which is what you want when a
+tap is retried on a flaky mobile connection. What bounds it is scope, not a use
+count: one incident, one action, one day. Expired tokens are purged by the
+sweeper.
 
 Two consequences worth planning for:
 
@@ -281,7 +288,7 @@ Authorization: Bearer <api-key>
 
 | Method | Path | Description |
 |---|---|---|
-| `POST` | `/api/notify/ack/{token}` | Acknowledge an incident from a push notification's Acknowledge button. No auth: the single-use token in the path is the credential. Must stay publicly reachable |
+| `POST` | `/api/notify/ack/{token}` | Acknowledge an incident from a push notification's Acknowledge button. No auth: the token in the path is the credential — one incident, one action, 24 hours, idempotent. Must stay publicly reachable |
 
 ### Incidents
 

charts/terdut-server/values.yaml

@@ -42,7 +42,7 @@ notify:
   #
   # The Acknowledge button is a POST to /api/notify/ack/{token} from the
   # responder's phone, so that path has to stay publicly reachable — it is
-  # authorised by the single-use token in the URL, not by network placement.
+  # authorised by the scoped token in the URL, not by network placement.
   publicUrl: ""
   # Optional bearer token for an access-controlled ntfy, read from an existing
   # Secret. Leave name empty for an open ntfy.

internal/api/router.go

@@ -22,7 +22,7 @@ func NewRouter(db *sql.DB, notify NotifyConfig) http.Handler {
 
 	// Unauthenticated: bootstrap, the Alertmanager webhook receiver, and the
 	// Acknowledge button in a push notification. The last one is authorised by
-	// the single-use token in its path rather than an API key, and has to stay
+	// the scoped token in its path rather than an API key, and has to stay
 	// reachable from outside the cluster for the button to work.
 	r.Post("/api/bootstrap", handleBootstrap(db))
 	r.Post("/api/alertmanager/webhook", handleAlertmanagerWebhook(db, notify))