Support selectable demo exchanges and configurable analysis limits

Persist manual requests until exchange confirmation, preserve stop protection, and reconcile fills before settling plans. Tighten model stream, crawler, skill, research accounting, and frontend race handling, with regression coverage. Keep queued analyses pending until a fresh account snapshot is available.

backend/src/alphapulse/trading/engine.py

@@ -15,7 +15,7 @@ from alphapulse.trading.analyst import run_analysis
 from alphapulse.trading.executor import executor_loop
 from alphapulse.trading.news_signals import news_signal_loop
 from alphapulse.trading.schedule import routine_requests
-from alphapulse.trading.snapshot import snapshot_loop
+from alphapulse.trading.snapshot import SnapshotError, read_snapshot, require_fresh_snapshot, snapshot_loop
 
 logger = logging.getLogger("alphapulse.trading.engine")
 
@@ -36,6 +36,11 @@ async def fail_interrupted() -> None:
 
 async def claim_requests(limit: int, busy: set[str]) -> list[tuple[str, list[str]]]:
     """取出到期的分析请求并清空，同一请求里的多个触发原因合并为一次分析"""
+    try:
+        require_fresh_snapshot(await read_snapshot())
+    except SnapshotError:
+        # 领取后就会创建分析记录；账户未就绪时保留请求，避免空耗次数和分析间隔。
+        return []
     async with SessionLocal() as session:
         symbols = (await session.scalars(
             select(TradingSymbol)