feat: add peer user ID to DM conversation responses

- Introduced a new field `peer_user_id` in the API documentation and conversation responses for direct messages (DMs), allowing clients to identify the counterpart user in a conversation.
- Updated the HTTP dispatch logic to include `peer_user_id` when handling DM conversations.
- Enhanced unit tests to verify the presence of `peer_user_id` in DM responses and ensure it is omitted for group conversations.

Author: bialger

API.md

@@ -1190,6 +1190,7 @@ Each element of `conversations`:
 | `type`               | integer         | `0` DM, `1` group, `2` channel |
 | `created_by`         | string          | User id     |
 | `created_by_username` | string or null | Canonical username for `created_by` (non-disabled); `null` if not available |
+| `peer_user_id`       | string          | Present only for DM (`type = 0`): counterpart user id relative to caller; for self-DM equals caller `user_id` |
 | `created_at`         | integer         | Unix seconds (conversation created) |
 | `membership_version` | integer         | Bumps on membership changes |
 | `last_activity_at`   | integer or null | Unix seconds of the latest stored envelope in this conversation (`MAX(server_timestamp)`); `null` if there are no envelopes yet |
@@ -1204,6 +1205,7 @@ Each element of `conversations`:
       "type": 0,
       "created_by": "usr_alice",
       "created_by_username": "alice",
+      "peer_user_id": "usr_bob",
       "created_at": 1710000000,
       "membership_version": 3,
       "last_activity_at": 1710000123
@@ -1240,6 +1242,7 @@ Common fields:
 | `type`               | integer | `0` DM, `1` group, `2` channel |
 | `created_by`         | string  |             |
 | `created_by_username` | string or null | Canonical username for `created_by` |
+| `peer_user_id`       | string  | Present only for DM (`type = 0`): counterpart user id relative to caller; for self-DM equals caller `user_id` |
 | `created_at`         | integer | Unix seconds |
 | `membership_version` | integer |             |
 | `my_role`            | string  | `"owner"`, `"admin"`, or `"member"` |
@@ -1251,7 +1254,8 @@ Additional fields for **channel** (`type == 2`) only:
 | `title`                 | string | Channel title (may be empty) |
 | `channel_post_policy`   | string | e.g. `admins_only` or policy serialized by server |
 
-**200 (DM or group)** — `title` and `channel_post_policy` are **not** returned; only `my_role` among the extra fields below.
+**200 (DM or group)** — `title` and `channel_post_policy` are **not** returned; only `my_role` among the extra fields below.  
+For DM (`type = 0`) `peer_user_id` is included; for group (`type = 1`) it is omitted.
 
 ```json
 {

lib/vox_net/http_dispatch.cpp

@@ -109,6 +109,21 @@ void SetProfileField(boost::json::object& o,
   }
 }
 
+std::optional<vox::common::UserId> DmPeerUserId(const std::vector<vox::store::MemberRecord>& members,
+                                                const vox::common::UserId& self_user_id) {
+  for (const auto& member : members) {
+    if (member.user_id != self_user_id) {
+      return member.user_id;
+    }
+  }
+  for (const auto& member : members) {
+    if (member.user_id == self_user_id) {
+      return member.user_id;
+    }
+  }
+  return std::nullopt;
+}
+
 common::Timestamp NowSeconds() {
   return std::chrono::duration_cast<std::chrono::seconds>(std::chrono::system_clock::now().time_since_epoch()).count();
 }
@@ -1113,6 +1128,14 @@ OptRes HandleAuthenticated(ServerContext& ctx,
         }
         out["created_at"] = conv->created_at;
         out["membership_version"] = conv->membership_version;
+        if (conv->type == common::ConversationType::kDm) {
+          const auto members = ctx.conversations_store.GetMembers(*conv_only);
+          if (const auto peer = DmPeerUserId(members, sess.user_id)) {
+            out["peer_user_id"] = *peer;
+          } else {
+            out["peer_user_id"] = nullptr;
+          }
+        }
         if (conv->type == common::ConversationType::kChannel) {
           if (pol.contains("title")) {
             out["title"] = pol.at("title");
@@ -1349,6 +1372,14 @@ OptRes HandleAuthenticated(ServerContext& ctx,
           SetProfileField(co, creator_names, c.created_by, "created_by_username");
           co["created_at"] = c.created_at;
           co["membership_version"] = c.membership_version;
+          if (c.type == common::ConversationType::kDm) {
+            const auto members = ctx.conversations_store.GetMembers(c.conversation_id);
+            if (const auto peer = DmPeerUserId(members, sess.user_id)) {
+              co["peer_user_id"] = *peer;
+            } else {
+              co["peer_user_id"] = nullptr;
+            }
+          }
           if (c.last_activity_at) {
             co["last_activity_at"] = *c.last_activity_at;
           } else {

tests/net_api_tests.cpp

@@ -206,6 +206,8 @@ TEST_F(NetApiTestSuite, ListConversationsIncludesCreatedByUsername) {
     const auto& o = item.as_object();
     if (o.at("conversation_id").as_string() == conv_id) {
       ASSERT_EQ(o.at("created_by_username").as_string(), "lst_b");
+      ASSERT_TRUE(o.contains("peer_user_id"));
+      ASSERT_EQ(o.at("peer_user_id").as_string(), a.user_id);
       found = true;
       break;
     }
@@ -226,6 +228,36 @@ TEST_F(NetApiTestSuite, GetConversationIncludesCreatedByUsername) {
   ASSERT_EQ(gst, 200u);
   auto o = ParseObj(gbody);
   ASSERT_EQ(o["created_by_username"].as_string(), "gone_b");
+  ASSERT_TRUE(o.contains("peer_user_id"));
+  ASSERT_EQ(o["peer_user_id"].as_string(), a.user_id);
+}
+
+TEST_F(NetApiTestSuite, ListConversationsOmitsPeerUserIdForGroup) {
+  auto a = RegisterUser("grp_l_a", "grp_lda");
+  auto b = RegisterUser("grp_l_b", "grp_ldb");
+  boost::json::object conv;
+  conv["type"] = "group";
+  boost::json::array members;
+  members.emplace_back(a.user_id);
+  members.emplace_back(b.user_id);
+  conv["members"] = members;
+  auto [cst, cbody] = HttpPost("/v1/conversations", boost::json::serialize(conv), a.access_token);
+  ASSERT_EQ(cst, 200u);
+  std::string conv_id = JsonString(cbody, "conversation_id");
+
+  auto [gst, gbody] = HttpGet("/v1/conversations", a.access_token);
+  ASSERT_EQ(gst, 200u);
+  bool found = false;
+  auto list_root = ParseObj(gbody);
+  for (const auto& item : list_root["conversations"].as_array()) {
+    const auto& o = item.as_object();
+    if (o.at("conversation_id").as_string() == conv_id) {
+      ASSERT_FALSE(o.contains("peer_user_id"));
+      found = true;
+      break;
+    }
+  }
+  ASSERT_TRUE(found);
 }
 
 TEST_F(NetApiTestSuite, LogoutRevokesBearer) {
@@ -691,6 +723,11 @@ TEST_F(NetApiTestSuite, SelfDmCreateAndSendMessage) {
   auto [cst, cbody] = HttpPost("/v1/conversations", boost::json::serialize(conv), u.access_token);
   ASSERT_EQ(cst, 200u);
   std::string conv_id = JsonString(cbody, "conversation_id");
+  auto [gst, gbody] = HttpGet(std::string("/v1/conversations/") + conv_id, u.access_token);
+  ASSERT_EQ(gst, 200u);
+  auto conv_obj = ParseObj(gbody);
+  ASSERT_TRUE(conv_obj.contains("peer_user_id"));
+  ASSERT_EQ(conv_obj["peer_user_id"].as_string(), u.user_id);
 
   boost::json::object send;
   send["device_id"] = "dev_self";