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/__init__.py

@@ -0,0 +1,44 @@
+"""Shared SQLAlchemy models — import all models here so Alembic can discover them."""
+
+from shared.models.base import Base, TimestampMixin
+from shared.models.trading import (
+    Signal,
+    SignalDirection,
+    Strategy,
+    StrategyWeightHistory,
+    Trade,
+    TradeSide,
+    TradeStatus,
+    Position,
+)
+from shared.models.news import Article, ArticleSentiment
+from shared.models.learning import LearningAdjustment, TradeOutcome
+from shared.models.auth import User, UserCredential
+from shared.models.timeseries import MarketData, PortfolioSnapshot, StrategyMetric
+
+__all__ = [
+    "Base",
+    "TimestampMixin",
+    # Trading
+    "Strategy",
+    "Signal",
+    "SignalDirection",
+    "Trade",
+    "TradeSide",
+    "TradeStatus",
+    "Position",
+    "StrategyWeightHistory",
+    # News
+    "Article",
+    "ArticleSentiment",
+    # Learning
+    "TradeOutcome",
+    "LearningAdjustment",
+    # Auth
+    "User",
+    "UserCredential",
+    # Timeseries
+    "MarketData",
+    "PortfolioSnapshot",
+    "StrategyMetric",
+]