fix: enable cross-job portfolio continuity in get_starting_holdings

Remove job_id filter from get_starting_holdings() SQL JOIN to enable
holdings continuity across jobs. This completes the cross-job portfolio
continuity fix started in the previous commit.

Root cause: get_starting_holdings() joined on job_id, preventing it from
finding previous day's holdings when queried from a different job. This
caused starting_position.holdings to be empty in API results for new jobs
even though starting_cash was correctly retrieved.

Changes:
- api/database.py: Remove job_id from JOIN condition in get_starting_holdings()
- tests/unit/test_database_helpers.py: Add test for cross-job holdings retrieval

Together with the previous commit fixing get_previous_trading_day(), this
ensures complete portfolio continuity (both cash and holdings) across jobs.

api/database.py

@@ -661,6 +661,9 @@ class Database:
     def get_starting_holdings(self, trading_day_id: int) -> list:
         """Get starting holdings from previous day's ending holdings.
 
+        NOTE: Queries across ALL jobs for the given model to enable portfolio
+        continuity even when new jobs are created with overlapping date ranges.
+
         Returns:
             List of dicts with keys: symbol, quantity
             Empty list if first trading day
@@ -671,7 +674,6 @@ class Database:
             SELECT td_prev.id
             FROM trading_days td_current
             JOIN trading_days td_prev ON
-                td_prev.job_id = td_current.job_id AND
                 td_prev.model = td_current.model AND
                 td_prev.date < td_current.date
             WHERE td_current.id = ?