feat(DATA-DB-002): add BacktestSettings model and repository

- BacktestSettings dataclass with all configuration fields
- BacktestSettingsRepository for persistence per workspace
- Settings independent of portfolio configuration
- Full validation for dates, symbols, LTV, etc.
- 16 comprehensive tests

Fields:
- settings_id, name: identification
- data_source: databento|yfinance|synthetic
- dataset, schema: Databento configuration
- start_date, end_date: date range
- underlying_symbol, start_price, underlying_units: position config
- loan_amount, margin_call_ltv: LTV analysis
- template_slugs: strategies to test
- cache_key, data_cost_usd: caching metadata
- provider_ref: provider configuration

app/models/backtest_settings.py

@@ -0,0 +1,88 @@
+"""Backtest settings model for configuring backtest scenarios independently of portfolio settings."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from datetime import date
+from typing import Literal
+from uuid import UUID, uuid4
+
+# Self type annotation
+from app.models.backtest import ProviderRef
+
+
+@dataclass(frozen=True)
+class BacktestSettings:
+    """Configuration for running backtests independent of portfolio settings.
+
+    These settings determine what data to fetch and how to run backtests,
+    separate from the actual portfolio configurations being tested.
+    """
+
+    settings_id: UUID
+    name: str
+    data_source: Literal["databento", "yfinance", "synthetic"]
+    dataset: str
+    schema: str
+    start_date: date
+    end_date: date
+    underlying_symbol: Literal["GLD", "GC", "XAU"]
+    start_price: float
+    underlying_units: float
+    loan_amount: float
+    margin_call_ltv: float
+    template_slugs: tuple[str, ...]
+    cache_key: str
+    data_cost_usd: float
+    provider_ref: ProviderRef
+
+    def __post_init__(self) -> None:
+        if not self.settings_id:
+            raise ValueError("settings_id is required")
+        if not self.name:
+            raise ValueError("name is required")
+        if self.start_date > self.end_date:
+            raise ValueError("start_date must be on or before end_date")
+        if self.data_source not in ("databento", "yfinance", "synthetic"):
+            raise ValueError("data_source must be 'databento', 'yfinance', or 'synthetic'")
+        if self.underlying_symbol not in ("GLD", "GC", "XAU"):
+            raise ValueError("underlying_symbol must be 'GLD', 'GC', or 'XAU'")
+        if self.start_price < 0:
+            raise ValueError("start_price must be non-negative")
+        if self.underlying_units <= 0:
+            raise ValueError("underlying_units must be positive")
+        if self.loan_amount < 0:
+            raise ValueError("loan_amount must be non-negative")
+        if not 0 < self.margin_call_ltv < 1:
+            raise ValueError("margin_call_ltv must be between 0 and 1")
+        if not self.template_slugs:
+            raise ValueError("at least one template slug is required")
+        if self.data_cost_usd < 0:
+            raise ValueError("data_cost_usd must be non-negative")
+
+    @classmethod
+    def create_default(cls, name: str = "Default Backtest Settings") -> BacktestSettings:
+        """Create default backtest settings configuration."""
+        return cls(
+            settings_id=uuid4(),
+            name=name,
+            data_source="databento",
+            dataset="XNAS.BASIC",
+            schema="ohlcv-1d",
+            start_date=date(2020, 1, 1),
+            end_date=date(2023, 12, 31),
+            underlying_symbol="GLD",
+            start_price=0.0,
+            underlying_units=1000.0,
+            loan_amount=0.0,
+            margin_call_ltv=0.75,
+            template_slugs=("default-template",),
+            cache_key="",
+            data_cost_usd=0.0,
+            provider_ref=ProviderRef(provider_id="default", pricing_mode="standard"),
+        )
+
+
+# For backward compatibility - alias to existing models
+BacktestScenario = "app.models.backtest.BacktestScenario"
+# TemplateRef and ProviderRef imported from app.models.backtest