test: add end-to-end test for complete simulation workflow

- Created comprehensive E2E test in tests/e2e/test_full_simulation_workflow.py
- Tests new trading_days schema with manually populated data
- Verifies database helper methods work correctly
- Tests Results API structure and filtering
- Validates holdings chain across multiple days
- Checks daily P&L calculation and storage
- Verifies reasoning summary/full retrieval
- Fixed database index creation for backward compatibility with old schema
- Added migration script for cleaning old positions table
- Test uses dependency override to ensure API uses correct database

NOTE: Test does not run full simulation since model_day_executor
has not yet been migrated to new schema. Instead directly populates
trading_days table and validates API layer works correctly.

Test verifies Task 9 requirements from implementation plan.

scripts/migrate_clean_database.py

@@ -0,0 +1,69 @@
+#!/usr/bin/env python3
+"""
+Clean database migration script.
+
+Drops old positions table and creates fresh trading_days schema.
+WARNING: This deletes all existing position data.
+"""
+
+import sys
+import os
+
+# Add parent directory to path
+sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+from api.database import Database
+import importlib.util
+import sys
+
+# Import migration module using importlib to handle numeric prefix
+spec = importlib.util.spec_from_file_location(
+    "trading_days_schema",
+    os.path.join(os.path.dirname(os.path.dirname(os.path.abspath(__file__))),
+                 "api", "migrations", "001_trading_days_schema.py")
+)
+trading_days_schema = importlib.util.module_from_spec(spec)
+spec.loader.exec_module(trading_days_schema)
+drop_old_positions_table = trading_days_schema.drop_old_positions_table
+
+
+def migrate_clean_database():
+    """Drop old schema and create clean new schema."""
+    print("Starting clean database migration...")
+
+    db = Database()
+
+    # Drop old positions table
+    print("Dropping old positions table...")
+    drop_old_positions_table(db)
+
+    # New schema already created by Database.__init__()
+    print("New trading_days schema created successfully")
+
+    # Verify new tables exist
+    cursor = db.connection.execute(
+        "SELECT name FROM sqlite_master WHERE type='table' ORDER BY name"
+    )
+    tables = [row[0] for row in cursor.fetchall()]
+
+    print(f"\nCurrent tables: {', '.join(tables)}")
+
+    # Verify positions table is gone
+    if 'positions' in tables:
+        print("WARNING: positions table still exists!")
+        return False
+
+    # Verify new tables exist
+    required_tables = ['trading_days', 'holdings', 'actions']
+    for table in required_tables:
+        if table not in tables:
+            print(f"ERROR: Required table '{table}' not found!")
+            return False
+
+    print("\nMigration completed successfully!")
+    return True
+
+
+if __name__ == "__main__":
+    success = migrate_clean_database()
+    sys.exit(0 if success else 1)