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

alembic/script.py.mako

@@ -0,0 +1,28 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    ${downgrades if downgrades else "pass"}