Backport to 4.83.0: Use swap table pattern and batch delete to improv…

[JordanMontgomery]

…e DB access patterns for vuln cron (#41729)

Related issue: Resolves #41664

Switching kernel_host_counts to the established swap pattern. Reduce load on the DB writer by moving the large read to the DB reader.

Do CleanupSoftwareTitles in batches. With a single large select/delete, it took > 16 minutes. In batches, it took ~1.5 minutes in loadtest with 100K hosts.

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.

• QA'd all new/changed functionality manually

For unreleased bug fixes in a release candidate, one of:

• Alerted the release DRI if additional load testing is needed

• Performance Improvements

• Added indexing and a batched swap/insert flow to speed up vulnerability-related queries and lower maintenance contention.

• Batched cleanup of orphaned records to reduce long-running delete operations.

• Reliability

• Migration removes a legacy constraint to simplify data maintenance and avoid migration failures.

• Scheduled vulnerability refresh now runs more atomically to reduce disruption.

• Tests

• Updated assertion logic to improve test clarity for host-count verification.

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[codecov]

Codecov Report

❌ Patch coverage is 70.08547% with 35 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (rc-minor-fleet-v4.83.0@26c1a97). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...atastore/mysql/operating_system_vulnerabilities.go	66.21%	16 Missing and 9 partials ⚠️
server/datastore/mysql/software.go	82.85%	3 Missing and 3 partials ⚠️
...s/20260316120000_DropKernelHostCountsForeignKey.go	50.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##             rc-minor-fleet-v4.83.0   #42300   +/-   ##
=========================================================
  Coverage                          ?   66.40%           
=========================================================
  Files                             ?     2503           
  Lines                             ?   200948           
  Branches                          ?     8872           
=========================================================
  Hits                              ?   133442           
  Misses                            ?    55441           
  Partials                          ?    12065           

Flag	Coverage Δ
backend	68.17% <70.08%> (?)

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