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/db.py

@@ -0,0 +1,22 @@
+"""SQLAlchemy async engine and session factory."""
+
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker, create_async_engine
+
+from shared.config import BaseConfig
+
+
+def create_db(config: BaseConfig) -> tuple:
+    """Create an async engine and session factory from the given config.
+
+    Returns a ``(engine, session_factory)`` tuple.
+    """
+    engine = create_async_engine(
+        config.database_url,
+        echo=config.log_level == "DEBUG",
+    )
+    session_factory = async_sessionmaker(
+        engine,
+        class_=AsyncSession,
+        expire_on_commit=False,
+    )
+    return engine, session_factory