Add trading engine, UI dashboard, and entry point stubs

- Add main.py: Async trading engine with regime detection, Supertrend
  strategy, position sizing, and order execution with retry logic
- Add app.py: Streamlit dashboard for monitoring (placeholder UI)
- Add backtest.py: Stub entry point for tf-backtest CLI
- Add optimize.py: Stub entry point for tf-optimize CLI
- Update package __init__.py files with version and docstrings

All quality checks pass:
- ruff check: 0 errors
- ruff format: formatted
- mypy src/: 0 errors (strict)
- pytest: 191 passed, 68% coverage

src/tradefinder/core/optimize.py

@@ -0,0 +1,21 @@
+"""Stub optimizer entry point; real logic coming with Optuna."""
+
+import asyncio
+
+import structlog
+
+from tradefinder.core.config import get_settings
+
+logger = structlog.get_logger(__name__)
+
+
+async def run_optimizer() -> None:
+    """Placeholder async optimizer run; future Optuna logic belongs here."""
+    settings = get_settings()
+    # TODO: add Optuna study setup and walk-forward execution pipeline.
+    logger.info("Optimizer not yet implemented", trading_mode=settings.trading_mode.value)
+
+
+def main() -> None:
+    """Sync wrapper so python -m tradefinder.core.optimize works."""
+    asyncio.run(run_optimizer())