feat: database models and alembic migrations — all tables per design

- shared/db.py: async engine + session factory
- shared/models/base.py: DeclarativeBase + TimestampMixin
- shared/models/trading.py: Strategy, Signal, Trade, Position, StrategyWeightHistory
- shared/models/news.py: Article, ArticleSentiment
- shared/models/learning.py: TradeOutcome, LearningAdjustment
- shared/models/auth.py: User, UserCredential
- shared/models/timeseries.py: MarketData, PortfolioSnapshot, StrategyMetric
- Alembic async env.py with initial migration including TimescaleDB hypertables
- 21 model tests covering enums, instantiation, metadata registration

shared/models/auth.py

@@ -0,0 +1,39 @@
+"""Authentication models: User, UserCredential."""
+
+import uuid
+
+from sqlalchemy import ForeignKey, Integer, LargeBinary, String
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from shared.models.base import Base, TimestampMixin
+
+
+class User(TimestampMixin, Base):
+    __tablename__ = "users"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    username: Mapped[str] = mapped_column(String(100), unique=True, nullable=False)
+    display_name: Mapped[str | None] = mapped_column(String(255), nullable=True)
+
+    # Relationships
+    credentials: Mapped[list["UserCredential"]] = relationship(back_populates="user")
+
+
+class UserCredential(TimestampMixin, Base):
+    __tablename__ = "user_credentials"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("users.id"), nullable=False
+    )
+    credential_id: Mapped[str] = mapped_column(String(512), unique=True, nullable=False)
+    public_key: Mapped[bytes] = mapped_column(LargeBinary, nullable=False)
+    sign_count: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
+
+    # Relationships
+    user: Mapped[User] = relationship(back_populates="credentials")