feat: add new day-centric results API endpoint

Implements new /results endpoint with day-centric data structure:
- Returns starting_position, daily_metrics, trades, and final_position
- Supports reasoning levels: none (default), summary, full
- Uses database helper methods from trading_days schema
- Replaces old positions-based endpoint

Changes:
- Created api/routes/results_v2.py with new endpoint
- Registered router in api/main.py
- Removed old /results endpoint (positions table)
- Added comprehensive integration tests

All tests pass.

api/routes/__init__.py

@@ -0,0 +1 @@
+"""API routes package."""