feat: pydantic schemas for all service message types

- shared/schemas/trading.py: OrderRequest, OrderResult, PositionInfo,
  AccountInfo, TradeSignal, TradeExecution, MarketSnapshot, SentimentContext
- shared/schemas/news.py: RawArticle, ScoredArticle
- shared/schemas/learning.py: TradeOutcomeSchema, WeightAdjustment
- shared/schemas/auth.py: RegisterRequest, LoginRequest, TokenResponse
- 49 schema tests covering validation constraints, serialization round-trips,
  required fields, and range checks

shared/schemas/__init__.py

@@ -0,0 +1,37 @@
+"""Pydantic v2 schemas for all service message types."""
+
+from shared.schemas.trading import (
+    AccountInfo,
+    MarketSnapshot,
+    OrderRequest,
+    OrderResult,
+    PositionInfo,
+    SentimentContext,
+    TradeExecution,
+    TradeSignal,
+)
+from shared.schemas.news import RawArticle, ScoredArticle
+from shared.schemas.learning import TradeOutcomeSchema, WeightAdjustment
+from shared.schemas.auth import LoginRequest, RegisterRequest, TokenResponse
+
+__all__ = [
+    # Trading
+    "OrderRequest",
+    "OrderResult",
+    "PositionInfo",
+    "AccountInfo",
+    "TradeSignal",
+    "TradeExecution",
+    "MarketSnapshot",
+    "SentimentContext",
+    # News
+    "RawArticle",
+    "ScoredArticle",
+    # Learning
+    "TradeOutcomeSchema",
+    "WeightAdjustment",
+    # Auth
+    "RegisterRequest",
+    "LoginRequest",
+    "TokenResponse",
+]