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/news.py

@@ -0,0 +1,49 @@
+"""News and sentiment models: Article, ArticleSentiment."""
+
+import uuid
+from datetime import datetime
+
+from sqlalchemy import DateTime, Float, ForeignKey, String, Text
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+
+from shared.models.base import Base, TimestampMixin
+
+
+class Article(TimestampMixin, Base):
+    __tablename__ = "articles"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    source: Mapped[str] = mapped_column(String(100), nullable=False)
+    url: Mapped[str] = mapped_column(Text, nullable=False)
+    title: Mapped[str] = mapped_column(Text, nullable=False)
+    published_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    fetched_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), nullable=False
+    )
+    content_hash: Mapped[str] = mapped_column(String(64), unique=True, nullable=False)
+
+    # Relationships
+    sentiments: Mapped[list["ArticleSentiment"]] = relationship(back_populates="article")
+
+
+class ArticleSentiment(TimestampMixin, Base):
+    __tablename__ = "article_sentiments"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    article_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("articles.id"), nullable=False
+    )
+    ticker: Mapped[str] = mapped_column(String(20), nullable=False, index=True)
+    score: Mapped[float] = mapped_column(Float, nullable=False)
+    confidence: Mapped[float] = mapped_column(Float, nullable=False)
+    model_used: Mapped[str] = mapped_column(String(50), nullable=False)
+
+    # Relationships
+    article: Mapped[Article] = relationship(back_populates="sentiments")