fix(chat): address adversarial review of the send-path change

Four confirmed findings from the review of ef247c9:

1. Persistence regression — the deferred-persist echo (in-memory Sending, written
   only when the async callback resolved) meant a message broadcast on-chain but
   whose callback hadn't fired yet was LOST from history if the app quit/crashed
   in that window. Persist the echo immediately as Sending and UPSERT the final
   status on resolve (new ChatDatabase::upsert with ON CONFLICT DO UPDATE, since
   append is INSERT-OR-IGNORE). A stray persisted Sending still loads as Sent.

2. Fee ceiling — dragonxd REJECTS a 0-value tx whose fee exceeds the default
   miners fee (0.0001), and max(getDefaultFee(), 0.0001) can only raise it, so a
   default_fee > 0.0001 broke every chat send. Pin chat to exactly kChatMinFeeDrgx,
   dropping getDefaultFee() from this path (chat always moves 0 value).

3. Lifetime — the resolve callback had no generation guard, so a wallet lock (which
   doesn't disconnect) between submit and callback could resolve against a cleared
   store. Capture chat_session_generation_ and bail on mismatch (both the full-node
   callback and the lite optimistic resolve), matching the identity-fetch pattern.

4. Retry misdirect — Retry on a failed CONTACT REQUEST called sendChatMessage,
   which (no peer key yet) just showed "waiting for reply". Route it to
   sendContactRequestForCid() (refactored out of startChatConversation) so it
   re-sends the request into the SAME conversation.

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

src/chat/chat_database.cpp

@@ -129,6 +129,35 @@ bool ChatDatabase::append(const ChatMessage& message)
     return sqlite3_changes(db_) > 0;
 }
 
+bool ChatDatabase::upsert(const ChatMessage& message)
+{
+    if (!key_ready_ || !ensureOpen()) return false;
+
+    std::vector<unsigned char> nonce;
+    std::vector<unsigned char> cipher;
+    std::string plain = serialize(message);
+    const bool encrypted = encrypt(plain, nonce, cipher);
+    if (!plain.empty()) sodium_memzero(&plain[0], plain.size());
+    if (!encrypted) return false;
+
+    const std::string dedup = dedupHash(message.txid, message.payload_position);
+
+    sqlite3_stmt* stmt = nullptr;
+    if (sqlite3_prepare_v2(db_,
+            "INSERT INTO chat_messages (wallet_tag, dedup_hash, nonce, payload) VALUES (?, ?, ?, ?) "
+            "ON CONFLICT(wallet_tag, dedup_hash) DO UPDATE SET nonce=excluded.nonce, payload=excluded.payload",
+            -1, &stmt, nullptr) != SQLITE_OK) {
+        return false;
+    }
+    sqlite3_bind_text(stmt, 1, wallet_tag_.c_str(), -1, SQLITE_TRANSIENT);
+    sqlite3_bind_text(stmt, 2, dedup.c_str(), -1, SQLITE_TRANSIENT);
+    sqlite3_bind_blob(stmt, 3, nonce.data(), static_cast<int>(nonce.size()), SQLITE_TRANSIENT);
+    sqlite3_bind_blob(stmt, 4, cipher.data(), static_cast<int>(cipher.size()), SQLITE_TRANSIENT);
+    const bool done = sqlite3_step(stmt) == SQLITE_DONE;
+    sqlite3_finalize(stmt);
+    return done;
+}
+
 std::vector<ChatMessage> ChatDatabase::load()
 {
     std::vector<ChatMessage> out;