add a migration adding an index for claim task

[virajmehta]

Also added infra for nicer migrations

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Note

Improves claim_task performance and adds a migration to roll out new indexes across existing queues.

• Adds targeted indexes in ensure_queue_tables: r_*_ready partial index on (available_at, run_id) include (task_id), w_*_ti on task_id, r_*_cei partial index on claim_expires_at for running runs, t_*_cxlpol partial index on task_id for tasks with cancellation policies, and t_*_state_tid composite index on (state, task_id)
• Updates claim_task to only scan tasks that have cancellation policies when applying max_delay/max_duration cancellations
• Introduces migration 20251205000000_add_claim_task_indexes.sql to CREATE OR REPLACE affected functions and apply indexes to existing queues via a DO block

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