[0.9.1] FORK_IA_UX: Fase 2 cross-device — relay Karvan messages over SSB private msgs

Makes a Karvan room work between two different phones once they're connected to a pub:
- karvan_model: rooms track remoteFeeds (SSB ids of other participants); addRemoteFeed/
  getRemoteFeeds/anyRemoteRooms (feed-id validated, capped at MAX_MEMBERS).
- backend: inviting registers the invitee's feed; joining by link registers the inviter's
  feed (read from the invite PM). Posting a message publishes a private 'karvan-relay' to
  those feeds; an efficient LIVE log-stream subscriber (createLogStream old:false live:true)
  ingests incoming relays and injects them into the local mirror room (dedup by mid,
  skips own → no loop). WebRTC SIGNAL frames are NOT relayed (too many/slow for the log).
- Chosen over a muxrpc plugin because a classic pub only replicates the LOG (store-and-forward);
  live muxrpc wouldn't reach the other phone. No SSB-startup changes → no boot risk.
Verified: boot OK with the live stream, invite/adopt register feeds, message posts + relay
publishes, server stays up, 16/16 tests. Cross-device delivery needs the user's 2-phone+pub test.
Note: text chat only — video still needs the wrapper camera permission (Fase 3).

src/models/karvan_model.js

@@ -40,6 +40,7 @@ module.exports = ({ idleTtlMs = DEFAULT_IDLE_TTL_MS, absTtlMs = DEFAULT_ABS_TTL_
     room.messages.length = 0;
     room.signals.length = 0;
     room.members.clear();
+    room.remoteFeeds.clear();
     rooms.delete(id);
   };
 
@@ -62,6 +63,7 @@ module.exports = ({ idleTtlMs = DEFAULT_IDLE_TTL_MS, absTtlMs = DEFAULT_ABS_TTL_
       messages: [],
       signals: [],
       members: new Set(),
+      remoteFeeds: new Set(),   // feeds SSB de otros nodos con esta sala (para relay cross-device por privados)
       seq: 0,
       sigSeq: 0,
       idleTimer: null,
@@ -74,6 +76,17 @@ module.exports = ({ idleTtlMs = DEFAULT_IDLE_TTL_MS, absTtlMs = DEFAULT_ABS_TTL_
     return publicRoom(room);
   };
 
+  // Fase 2 (cross-device): registra el feed SSB de otro participante (de una invitación) para relayarle.
+  const addRemoteFeed = (id, feed) => {
+    const room = rooms.get(id);
+    if (!room || typeof feed !== "string" || !/^@[A-Za-z0-9+/=]{20,}\.ed25519$/.test(feed)) return false;
+    if (room.remoteFeeds.has(feed) || room.remoteFeeds.size < MAX_MEMBERS) { room.remoteFeeds.add(feed); return true; }
+    return false;
+  };
+  const getRemoteFeeds = (id) => { const r = rooms.get(id); return r ? [...r.remoteFeeds] : []; };
+  // ¿hay ALGUNA sala con feeds remotos? (para no montar el relay si no hace falta)
+  const anyRemoteRooms = () => { for (const r of rooms.values()) if (r.remoteFeeds.size) return true; return false; };
+
   // Fase 2 (invitar): unirse a una sala por su id creando un ESPEJO local (desde un enlace/invitación).
   // El id es la "capacidad": quien lo tiene puede unirse. Aún NO conecta con la sala del otro nodo
   // (eso es la señalización cross-device, pendiente); crea la sala local para poder abrir el enlace.
@@ -91,7 +104,7 @@ module.exports = ({ idleTtlMs = DEFAULT_IDLE_TTL_MS, absTtlMs = DEFAULT_ABS_TTL_
     if (Number.isFinite(t) && t > 0) absMs = Math.min(t * 60000, MAX_ABS_TTL_MS);
     const room = {
       id, title: String(title || "").slice(0, 80), createdAt, expiresAt: createdAt + absMs,
-      messages: [], signals: [], members: new Set(), seq: 0, sigSeq: 0, idleTimer: null, absTimer: null,
+      messages: [], signals: [], members: new Set(), remoteFeeds: new Set(), seq: 0, sigSeq: 0, idleTimer: null, absTimer: null,
     };
     rooms.set(id, room);
     room.absTimer = setTimeout(() => destroy(id), absMs);
@@ -171,6 +184,9 @@ module.exports = ({ idleTtlMs = DEFAULT_IDLE_TTL_MS, absTtlMs = DEFAULT_ABS_TTL_
   return {
     createRoom,
     adoptRoom,
+    addRemoteFeed,
+    getRemoteFeeds,
+    anyRemoteRooms,
     listRooms,
     getRoom,
     postMessage,