feat(chat): persistent, seed-encrypted message store (Phase 2)

Swap the in-memory-only chat store for durable sqlite persistence with real
per-transaction timestamps, encrypted at rest under a key derived from the
wallet's own seed (no passphrase, works on encrypted and unencrypted wallets).

- ChatDatabase (src/chat/chat_database.{h,cpp}): sqlite store mirroring
  data::TransactionHistoryCache. unlockWithSecret(seed) derives a 32-byte AEAD
  storage key and a wallet-partition tag via domain-separated keyed BLAKE2b
  (generichash) contexts. Every record — bodies, peer z-addrs, threading,
  timestamps — is crypto_aead_xchacha20poly1305_ietf-encrypted with a random
  nonce and the wallet tag as associated data; even the dedup key is a keyed
  hash of txid+position, so nothing about your conversations is in cleartext on
  disk. Rows are partitioned per-wallet; a different seed sees nothing. Messages
  are decrypted once at ingest then re-encrypted under the storage key, so
  load() needs only the storage key, not the chat identity.
- ChatService: ingest() now stamps each message with its own transaction time
  (txid->time map + fallback) and writes new (store-deduped) messages through to
  the database; loadFromDatabase() rehydrates the in-memory read model on unlock.
- App: unlock the chat DB with the same seed in provisionChatIdentityFromSecret
  and load prior history; lock the DB + clear the decrypted in-memory store on
  relock and on lite-controller rebuild.

Adversarial review (4 confirmed findings, all fixed): don't provision if the
wallet locks mid-fetch (re-check isLocked at completion); wipe the serialized
plaintext temporary in append(); trim the seed into a separate fully-wiped
buffer (no residue past a shrunk size()); scrub the mnemonic copy in the RPC
json result.

Tests: ChatDatabase round-trip (persist/reload, field + order fidelity), dedup,
per-wallet isolation, lock inertness, and ChatService write-through + reload
without an identity. Gated by DRAGONX_ENABLE_CHAT (default OFF). Verified:
Linux + Windows(mingw) build with chat ON, ctest 100%, hygiene clean; caches
restored to the OFF default.

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

src/app.cpp

@@ -604,8 +604,11 @@ void App::rebuildLiteWallet(bool force)
     lite_open_error_.clear();
 
     // A rebuilt controller may back a different wallet (server switch / re-open); drop any chat
-    // identity and re-arm provisioning so it re-derives from the newly opened wallet's seed.
+    // identity + decrypted messages, lock the DB, and re-arm provisioning so it re-derives (and
+    // reloads the right wallet's history) from the newly opened wallet's seed.
     chat_service_.clearIdentity();
+    chat_service_.store().clear();
+    chat_db_.lock();
     chat_identity_provisioned_ = false;
     chat_identity_fetch_in_flight_ = false;
     chat_identity_unavailable_ = false;