**Problem:**
Subsequent trading days were not retrieving starting holdings correctly.
The API showed empty starting_position and final_position even after
executing multiple buy trades.
**Root Cause:**
get_current_position_from_db() used `date <= ?` which returned the
CURRENT day's trading_day record instead of the PREVIOUS day's ending.
Since holdings are written at END of trading day, querying the current
day's record would return incomplete/empty holdings.
**Timeline on Day 1 (2025-10-02):**
1. Start: Create trading_day with empty holdings
2. Trade: Execute 8 buy trades (recorded in actions table)
3. End: Call get_current_position_from_db(date='2025-10-02')
- Query: `date <= 2025-10-02` returns TODAY's record
- Holdings: EMPTY (not written yet)
- Saves: Empty holdings to database ❌
**Solution:**
Changed query to use `date < ?` to retrieve PREVIOUS day's ending
position, which becomes the current day's starting position.
**Impact:**
- Day 1: Correctly saves ending holdings after trades
- Day 2+: Correctly retrieves previous day's ending as starting position
- Holdings now persist between trading days as expected
**Tests Added:**
- test_get_position_retrieves_previous_day_not_current: Verifies query
returns previous day when multiple days exist
- Updated existing tests to align with new behavior
Fixes holdings persistence bug identified in API response showing
empty starting_position/final_position despite successful trades.
