fix(relay): keep-alive on WebSocket, cap delivery attempts, honest receipts

Messages arrived late or not at all, for two independent reasons.

Dead connections: neither side sent a ping, and after AUTH the server
waited for a frame without a timeout. Mobile NAT drops idle TCP silently,
so the client wrote into a void while the relay still held the device in
its online map — and therefore never sent a push wake-up either. The ping
is a protocol-level one rather than v1::Ping, because dart:io answers
those on its own; the fix works with clients already in the wild.

Poisoned queue: a client decrypts an envelope (ratchet advances), stores
it and acknowledges it. If that ACK is lost, a second decryption attempt
can only fail, so the envelope is never acknowledged — and was redelivered
on every connect until its 30-day TTL, without ever reaching the recipient.
attempt_count existed from the first migration but was never counted; it
now bounds redelivery.

Receipts reported DELIVERED_ONLINE whenever a write into the bounded mpsc
channel succeeded, which it does even for a long-dead socket. Online is now
claimed only for devices that recently sent something.

Push send failures were logged at debug while the relay runs at info, so a
rejected wake-up looked exactly like a working one.

rust/sepot-relay/src/fcm.rs

@@ -143,11 +143,15 @@ impl FcmSender {
                 "android": { "priority": "HIGH" }
             }
         });
+        // warn, ne debug: relay běží na RUST_LOG=info, takže na debugu bylo
+        // selhání pushe neviditelné a tiše se hromadila fronta. Protistranou
+        // je Google, ne telefon uživatele — chyba tu znamená mrtvý token nebo
+        // rozbitou konfiguraci projektu, obojí je provozní problém.
         match http.post(&url).bearer_auth(access).json(&body).send().await {
             Ok(resp) if !resp.status().is_success() => {
-                tracing::debug!(status = %resp.status(), "FCM budíček odmítnut");
+                tracing::warn!(status = %resp.status(), "FCM budíček odmítnut");
             }
-            Err(e) => tracing::debug!("FCM budíček selhal: {e}"),
+            Err(e) => tracing::warn!("FCM budíček selhal: {e}"),
             _ => {}
         }
     }