feat(chat): message model, in-memory store, and receive service

Phase 1 Steps 4-6 (the receive pipeline, minus the App/sync wiring which
lands next).

- chat_message: the in-memory ChatMessage model (direction, kind, txid, cid,
  peer zaddr/pubkey, body, timestamp, payload_position). No libsodium.
- chat_store: threads messages by conversation_id and dedups by
  (txid, payload_position) so re-scanning the chain never double-inserts.
- chat_service: owns the long-lived chat identity keypair (move-disabled,
  wiped on destruction/clear) and the store. ingest() decrypts each Message
  (drops undecryptable ones silently — no plaintext/memo logging), passes a
  ContactRequest's plaintext through, and threads the result. No-op without an
  identity.

Tests: a metadata batch decrypts into a threaded conversation; re-ingest
dedups; a contact request carries through; a wrong identity decrypts nothing;
no-identity ingest is a no-op.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/chat/chat_service.cpp

@@ -0,0 +1,56 @@
+// DragonX Wallet - HushChat service (implementation).
+
+#include "chat_service.h"
+
+#include <utility>
+
+namespace dragonx::chat {
+
+ChatService::~ChatService() {
+    clearIdentity();
+}
+
+void ChatService::setIdentity(const ChatKeyPair& keys) {
+    identity_ = keys;
+    has_identity_ = true;
+}
+
+void ChatService::clearIdentity() {
+    wipeChatKeyPair(identity_);
+    has_identity_ = false;
+}
+
+int ChatService::ingest(const std::vector<HushChatTransactionMetadata>& metadata,
+                        std::int64_t txTimestamp) {
+    if (!has_identity_) return 0;
+
+    int added = 0;
+    for (const auto& meta : metadata) {
+        ChatMessage message;
+        message.direction = ChatDirection::Incoming;
+        message.txid = meta.txid;
+        message.conversation_id = meta.conversation_id;
+        message.peer_zaddr = meta.reply_zaddr;
+        message.peer_public_key_hex = meta.sender_public_key_hex;
+        message.timestamp = txTimestamp;
+        message.payload_position = meta.payload_position;
+
+        if (meta.type == HushChatHeaderType::ContactRequest) {
+            message.kind = ChatMessageKind::ContactRequest;
+            message.body = meta.payload_memo;  // plaintext request text
+        } else {
+            message.kind = ChatMessageKind::Message;
+            std::string plaintext;
+            const ChatCryptoStatus status = decryptIncoming(
+                identity_, meta.sender_public_key_hex, meta.secretstream_header_hex,
+                meta.payload_memo, plaintext);
+            if (status != ChatCryptoStatus::Ok) continue;  // drop undecryptable silently
+            message.body = std::move(plaintext);
+        }
+
+        if (store_.append(message)) ++added;
+    }
+    return added;
+}
+
+} // namespace dragonx::chat