Add demo trading analyst with plan validation and sizing

The analyst prompts the model with trading rules, published skills,
account sizing inputs and prefetched market data, exposes read-only
tools plus submit_trade_plan, and validates and sizes the submitted plan
in pure functions covered by pytest. Manual analyses run as dry runs
while trading is stopped; the trading page shows the latest plan and an
analysis detail drawer.

backend/pyproject.toml

@@ -27,3 +27,8 @@ dependencies = [
 [build-system]
 requires = ["uv_build>=0.12.23,<0.13.0"]
 build-backend = "uv_build"
+
+[dependency-groups]
+dev = [
+    "pytest>=9.1.1",
+]