feat: defer entry spot derivation to backtest run (BT-005)

- Remove async refresh_workspace_seeded_units from date change handlers
- Date changes now only call on_form_change() (updates cost estimates, marks results stale)
- Entry spot is derived only when user clicks Run button
- Form remains responsive during configuration
- No more API errors when changing dates while configuring other fields

docs/roadmap/backlog/BT-005-defer-entry-spot-derivation.yaml

@@ -1,22 +0,0 @@
-id: BT-005
-title: Defer Entry Spot Derivation to Backtest Run
-status: backlog
-priority: P1
-effort: S
-depends_on: []
-tags: [backtesting, ux, performance]
-summary: Only derive entry spot from historical data when user clicks Run, not on every form change.
-acceptance_criteria:
-  - Entry spot is NOT fetched when user changes start/end dates.
-  - Entry spot is NOT fetched when user changes other form fields.
-  - Entry spot IS derived when user clicks the Run button.
-  - Form remains responsive during date/field changes (no API calls blocking).
-  - Clear loading indicator shows when entry spot is being fetched during run.
-  - Previous entry spot value is retained until new one is derived.
-notes:
-  Current behavior calls derive_entry_spot on every date change which causes
-  API errors if user is still configuring other fields.
-  
-  The refresh_workspace_seeded_units function should not be called on date changes.
-  Entry spot derivation should happen inside start_backtest or as a separate
-  explicit "fetch spot" button if user wants to preview.