Fix newbook_current_rates column names + rollback on failed date

- schema defined gross_rate/net_rate but every query (fetch job,
  bookability, competitors) uses rate_gross/rate_net — rename the
  columns, with an idempotent DO-block migration for existing tables
- roll back the session when a date fails so one bad statement no
  longer poisons the whole sync run (every subsequent write was dying
  with 'current transaction is aborted')

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/jobs/fetch_current_rates.py

@@ -342,6 +342,9 @@ async def run_fetch_current_rates(horizon_days: int = 720, start_date: date = No
 
                 except Exception as e:
                     logger.warning(f"Failed to fetch rates for {current_date}: {e}")
+                    # A failed statement aborts the transaction — roll back so
+                    # subsequent dates in this run can still be written
+                    db.rollback()
 
                 # Step 5: Commit periodically
                 if day_count % COMMIT_BATCH_SIZE == 0: