Add index on task_reschedule ti_id

[guan404ming]

Related Issue

Closes #60895

Why

Deleting task instances is extremely slow (67+ seconds per row) because the ti_id foreign key column lacks an index. PostgreSQL performs sequential scans on task_reschedule when checking the FK constraint during cascading deletes.

How

Add index idx_task_reschedule_ti_id on task_reschedule(ti_id)

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