Backfill slack_integrations table in batches

What does this MR do and why?

This MR creates a batched background migration to backfill the sharding keys on the slack_integrations table.

References

#524680

Queries

SELECT
    "slack_integrations"."id"
FROM
    "slack_integrations"
WHERE
    "slack_integrations"."id" BETWEEN 23 AND 126
ORDER BY
    "slack_integrations"."id" ASC
LIMIT 1

 Limit  (cost=0.42..0.55 rows=1 width=4) (actual time=0.011..0.011 rows=1 loops=1)
   Buffers: shared hit=4
   I/O Timings: read=0.000 write=0.000
   ->  Index Only Scan using slack_integrations_pkey on public.slack_integrations  (cost=0.42..8.43 rows=63 width=4) (actual time=0.009..0.009 rows=1 loops=1)
         Index Cond: ((slack_integrations.id >= 23) AND (slack_integrations.id <= 126))
         Heap Fetches: 0
         Buffers: shared hit=4
         I/O Timings: read=0.000 write=0.000
Settings: effective_cache_size = '472585MB', jit = 'off', random_page_cost = '1.5', seq_page_cost = '4', work_mem = '100MB'

https://postgres.ai/console/gitlab/gitlab-production-main/sessions/45268/commands/138829

SELECT
    "slack_integrations"."id"
FROM
    "slack_integrations"
WHERE
    "slack_integrations"."id" BETWEEN 23 AND 126
    AND "slack_integrations"."id" >= 23
ORDER BY
    "slack_integrations"."id" ASC
LIMIT 1 OFFSET 1

 Limit  (cost=0.55..0.68 rows=1 width=4) (actual time=0.022..0.023 rows=1 loops=1)
   Buffers: shared hit=4
   I/O Timings: read=0.000 write=0.000
   ->  Index Only Scan using slack_integrations_pkey on public.slack_integrations  (cost=0.42..8.59 rows=63 width=4) (actual time=0.020..0.021 rows=2 loops=1)
         Index Cond: ((slack_integrations.id >= 23) AND (slack_integrations.id <= 126) AND (slack_integrations.id >= 23))
         Heap Fetches: 0
         Buffers: shared hit=4
         I/O Timings: read=0.000 write=0.000
Settings: seq_page_cost = '4', work_mem = '100MB', effective_cache_size = '472585MB', jit = 'off', random_page_cost = '1.5'

https://postgres.ai/console/gitlab/gitlab-production-main/sessions/45268/commands/138830

SELECT
    "slack_integrations"."integration_id"
FROM
    "slack_integrations"
WHERE
    "slack_integrations"."id" BETWEEN 23 AND 126
    AND "slack_integrations"."id" >= 23
    AND "slack_integrations"."id" < 24
    AND "slack_integrations"."project_id" IS NULL
    AND "slack_integrations"."group_id" IS NULL
    AND "slack_integrations"."organization_id" IS NULL

 Index Scan using slack_integrations_pkey on public.slack_integrations  (cost=0.42..3.45 rows=1 width=4) (actual time=0.004..0.004 rows=0 loops=1)
   Index Cond: ((slack_integrations.id >= 23) AND (slack_integrations.id <= 126) AND (slack_integrations.id >= 23) AND (slack_integrations.id < 24))
   Filter: ((slack_integrations.project_id IS NULL) AND (slack_integrations.group_id IS NULL) AND (slack_integrations.organization_id IS NULL))
   Rows Removed by Filter: 0
   Buffers: shared hit=3
   I/O Timings: read=0.000 write=0.000
Settings: effective_cache_size = '472585MB', jit = 'off', random_page_cost = '1.5', seq_page_cost = '4', work_mem = '100MB'

https://console.postgres.ai/gitlab/gitlab-production-main/sessions/45268/commands/138834

UPDATE
    "slack_integrations"
SET
    "updated_at" = '2025-11-06 13:26:57.176953',
    "organization_id" = 1
WHERE
    "slack_integrations"."id" = 23

 ModifyTable on public.slack_integrations  (cost=0.42..3.44 rows=0 width=0) (actual time=0.023..0.024 rows=0 loops=1)
   Buffers: shared hit=3
   I/O Timings: read=0.000 write=0.000
   ->  Index Scan using slack_integrations_pkey on public.slack_integrations  (cost=0.42..3.44 rows=1 width=22) (actual time=0.022..0.022 rows=0 loops=1)
         Index Cond: (slack_integrations.id = 23)
         Buffers: shared hit=3
         I/O Timings: read=0.000 write=0.000
Settings: random_page_cost = '1.5', seq_page_cost = '4', work_mem = '100MB', effective_cache_size = '472585MB', jit = 'off'

https://postgres.ai/console/gitlab/gitlab-production-main/sessions/45268/commands/138842

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