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
43067bb275