perf: Add composite index on jobs table for efficient job fetching

Add INDEX (status, priority, scheduled_time) to optimize the critical
populate query:

  SELECT ... FROM jobs
  WHERE status='pending' AND scheduled_time <= NOW()
  ORDER BY priority ASC, scheduled_time ASC

This index covers the WHERE clause and ORDER BY, eliminating table scans
and filesorts for large job queues.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/jobs.py

@@ -168,6 +168,7 @@ def _generate_definition(self) -> str:
     pid=0           : int32
     connection_id=0 : int64
     version=""      : varchar(64)
+    INDEX (status, priority, scheduled_time)
     """
 
     def _get_fk_derived_pk_attrs(self) -> list[tuple[str, str]]: