docs: add Databento integration plan and roadmap items

docs/roadmap/backlog/DATA-DB-002-backtest-settings-model.yaml

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+id: DATA-DB-002
+title: Backtest Settings Model
+status: backlog
+priority: high
+dependencies:
+  - DATA-DB-001
+estimated_effort: 1 day
+created: 2026-03-28
+updated: 2026-03-28
+
+description: |
+  Create BacktestSettings model that captures user-configurable backtest parameters
+  independent of portfolio settings. This allows running scenarios with custom start
+  prices and position sizes without modifying the main portfolio.
+
+acceptance_criteria:
+  - BacktestSettings dataclass defined with all necessary fields
+  - start_price can be 0 (auto-derive) or explicit value
+  - underlying_units independent of portfolio.gold_ounces
+  - loan_amount and margin_call_ltv for LTV analysis
+  - data_source field supports "databento" and "yfinance"
+  - Repository persists settings per workspace
+  - Default settings created for new workspaces
+
+implementation_notes: |
+  Key fields:
+  - settings_id: UUID for tracking
+  - data_source: "databento" | "yfinance" | "synthetic"
+  - dataset: "XNAS.BASIC" | "GLBX.MDP3"
+  - underlying_symbol: "GLD" | "GC" | "XAU"
+  - start_date, end_date: date range
+  - start_price: 0 for auto-derive, or explicit
+  - underlying_units: position size for scenario
+  - loan_amount: debt level for LTV analysis
+  
+  Settings are stored in .workspaces/{workspace_id}/backtest_settings.json
+
+dependencies_detail:
+  - DATA-DB-001: Need data source configuration fields