Add Binance demo trading key settings and signed demo client

Store demo API key/secret encrypted in a single-row trading_account table,
expose get/put/delete/test endpoints under /api/settings/exchange, and add
a settings card to enter and test the key. The client is hard-wired to
demo-fapi.binance.com.

backend/src/alphapulse/models/__init__.py

@@ -2,6 +2,7 @@ from alphapulse.core.db import Base
 from alphapulse.models.ai import AiProvider, ChatMessage, ChatSession, ContextVersion, SkillNode, SkillVersion
 from alphapulse.models.market import Kline, KlineSyncState, MarketSymbol
 from alphapulse.models.news import CrawlLog, CrawlRun, NewsItem, NewsRevision, RawBody, RawResponse, Source
+from alphapulse.models.trading import TradingAccount
 
 __all__ = [
     "AiProvider",
@@ -21,4 +22,5 @@ __all__ = [
     "SkillNode",
     "SkillVersion",
     "Source",
+    "TradingAccount",
 ]