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

AGENTS.md

@@ -1,4 +1,16 @@
 policy:
+  subagent_usage:
+    required: true
+    rules:
+      - prefer sub-agents for parallelizable implementation work
+      - use sub-agents when implementing independent roadmap items
+      - create worktrees for sub-agents working on the same codebase
+      - review sub-agent output before merging to main
+      - use `agent: implementation-reviewer` for code quality checks
+      - use `agent: qa-validator` for end-to-end validation
+      - chain sub-agents for multi-step workflows (plan → implement → review)
+      - always use sub-agents unless the task is trivial or requires direct interaction
+
   test_loop:
     required: true
     rules: