Optimize monitor deletion to prevent n+1 queries

[cursor]

Fixes N+1 queries and concurrency errors during Monitor deletion by batching seat removal.

The MonitorDeletionTask now calls quotas.backend.remove_seats with a list of monitors before individual deletion, centralizing and optimizing seat cleanup.

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. and is gonna need some rights from me in order to utilize my contributions in this here PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

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[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
29869	1	29868	245

View the top 1 failed test(s) by shortest run time

tests.sentry.deletions.test_monitor.DeleteMonitorTest::test_removes_monitor_seats_before_delete

Stack Traces | 2.03s run time

#x1B[1m#x1B[.../sentry/deletions/test_monitor.py#x1B[0m:70: in test_removes_monitor_seats_before_delete
    assert list(args[1]) == [monitor]
#x1B[1m#x1B[31mE   AssertionError: assert [<Monitor at ...001, name=''>] == [<Monitor at ...001, name=''>]#x1B[0m
#x1B[1m#x1B[31mE     #x1B[0m
#x1B[1m#x1B[31mE     At index 0 diff: <Monitor at 0x7f7f86668f50: id=None, guid=UUID('f38c3159-b8a8-49a0-b238-bab9f819b4b8'), project_id=4557137682432001, name=''> != <Monitor at 0x7f7f843bcb90: id=6, guid=UUID('f38c3159-b8a8-49a0-b238-bab9f819b4b8'), project_id=4557137682432001, name=''>#x1B[0m
#x1B[1m#x1B[31mE     #x1B[0m
#x1B[1m#x1B[31mE     Full diff:#x1B[0m
#x1B[1m#x1B[31mE       [#x1B[0m
#x1B[1m#x1B[31mE     -     <Monitor at 0x7f7f843bcb90: id=6, guid=UUID('f38c3159-b8a8-49a0-b238-bab9f819b4b8'), project_id=4557137682432001, name=''>,#x1B[0m
#x1B[1m#x1B[31mE     ?                        ^^^^^^      ^#x1B[0m
#x1B[1m#x1B[31mE     +     <Monitor at 0x7f7f86668f50: id=None, guid=UUID('f38c3159-b8a8-49a0-b238-bab9f819b4b8'), project_id=4557137682432001, name=''>,#x1B[0m
#x1B[1m#x1B[31mE     ?                        ^^^^^^      ^^^^#x1B[0m
#x1B[1m#x1B[31mE       ]#x1B[0m

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