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/auth.py

@@ -0,0 +1,27 @@
+"""Authentication Pydantic schemas for API request/response payloads."""
+
+from pydantic import BaseModel, Field
+
+
+class RegisterRequest(BaseModel):
+    """Sent by the dashboard to begin passkey registration."""
+
+    username: str = Field(min_length=1, max_length=100)
+    display_name: str | None = None
+
+
+class LoginRequest(BaseModel):
+    """Sent by the dashboard to begin passkey authentication."""
+
+    username: str = Field(min_length=1, max_length=100)
+
+
+class TokenResponse(BaseModel):
+    """Returned after successful authentication."""
+
+    access_token: str
+    refresh_token: str
+    token_type: str = "bearer"
+    expires_in: int = Field(
+        description="Access token lifetime in seconds", default=900
+    )