fix(chat): group-message FK violation + receipt-cursor stall

Group inserts were using recipient_id=0 as a "no recipient" sentinel,
which fails the tenants FK (foreign_keys is ON) and 500s on every
group send. Use the sender's own id as the placeholder instead;
group GET queries already key off room_uid so there's no self-message
artefact from the value.

Also exclude group rows from /api/chat/sent so the client's
last_sent_relay_id cursor doesn't stall on group sends that never
reach seen=1 (group receipts aren't tracked in v1).

Client-side: surface a clearer error when a room create hits a 404
on the relay — almost always means the relay is running pre-group
code and needs a restart.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

public/plugins/chat/backend.php

@@ -530,7 +530,8 @@ PluginManager::instance()->onApiRoute(function (string $route, string $method) {
 
         // Client-generated UID, same convention as collab buckets.
         $uid = bin2hex(random_bytes(8));
-        $ch = curl_init(rtrim($r['url'], '/') . '/api/chat/rooms');
+        $relayUrl = rtrim($r['url'], '/') . '/api/chat/rooms';
+        $ch = curl_init($relayUrl);
         curl_setopt_array($ch, [
             CURLOPT_RETURNTRANSFER => true,
             CURLOPT_POST => true,
@@ -547,7 +548,14 @@ PluginManager::instance()->onApiRoute(function (string $route, string $method) {
         curl_close($ch);
         if ($status !== 200) {
             $err = json_decode($response, true);
-            chatJson(['ok' => false, 'error' => $err['error'] ?? "relay HTTP $status"]);
+            $detail = $err['error'] ?? "relay HTTP $status";
+            // 404 on this path almost always means the relay is running a
+            // pre-group-chat build. Surface that explicitly so the user
+            // doesn't go hunting for data issues.
+            if ($status === 404) {
+                $detail = "relay does not know /api/chat/rooms — it's likely running old code. Restart the relay after pulling the latest commits.";
+            }
+            chatJson(['ok' => false, 'error' => $detail, 'url' => $relayUrl, 'relay_status' => $status]);
         }
         $data = json_decode($response, true) ?: [];
         // Mirror locally right away so the next conversations list shows

relay/inc/routes/chat.php

@@ -295,14 +295,15 @@ if ($route === '/api/chat' && $method === 'POST') {
     $stored = clientEncrypt($plain, $interKey);
 
     if ($roomUid !== '') {
-        // Group message: verify membership, single row with room_uid set
+        // Group message: verify membership. `recipient_id` is NOT NULL +
-        // and recipient_id=0 (sentinel — fan-out happens on GET via the
+        // has a tenants-FK, so we set it to the sender's own id as a
-        // chat_room_members join).
+        // placeholder — GET queries key off room_uid, never recipient_id,
+        // for group rows so this doesn't create a self-message artefact.
         $check = $db->prepare('SELECT 1 FROM chat_room_members WHERE room_uid = :r AND tenant_id = :t');
         $check->execute(['r' => $roomUid, 't' => $clientTenant['id']]);
         if (!$check->fetchColumn()) jsonResponse(['error' => 'not a member of this room'], 403);
-        $db->prepare('INSERT INTO chat_messages (recipient_id, sender_id, sender_username, room_uid, body, persistent) VALUES (0, :s, :sn, :ru, :b, 1)')
+        $db->prepare('INSERT INTO chat_messages (recipient_id, sender_id, sender_username, room_uid, body, persistent) VALUES (:me, :s, :sn, :ru, :b, 1)')
-            ->execute(['s' => $clientTenant['id'], 'sn' => $clientTenant['username'], 'ru' => $roomUid, 'b' => $stored]);
+            ->execute(['me' => $clientTenant['id'], 's' => $clientTenant['id'], 'sn' => $clientTenant['username'], 'ru' => $roomUid, 'b' => $stored]);
     } else {
         $recipient = getTenantByUsername($db, $recipientName);
         if (!$recipient) jsonResponse(['error' => "no such user: $recipientName"], 404);
@@ -463,10 +464,13 @@ if ($route === '/api/chat/backfill' && $method === 'GET') {
 if ($route === '/api/chat/sent' && $method === 'GET') {
     if (!$clientTenant) jsonResponse(['error' => 'unauthorized'], 401);
     $since = (int)($_GET['since'] ?? 0);
+    // 1:1 only — group messages use the same sender_id but have no
+    // per-recipient receipt state, so including them would stall the
+    // cursor on rows that never reach seen=1.
     $stmt = $db->prepare('
         SELECT id, recipient_id, fetched, seen, created_at
         FROM chat_messages
-        WHERE sender_id = :s AND id > :since
+        WHERE sender_id = :s AND room_uid IS NULL AND id > :since
         ORDER BY id ASC
         LIMIT 1000
     ');