Forms: Replace 3 count queries with single optimized endpoint

[lezama]

Summary

Replaces 3 separate REST API requests for status counts (inbox/spam/trash) with a single optimized database query.

Changes

• Added /wp/v2/feedback/counts REST endpoint with optimized single-query count logic
• Added caching (30s transient) for counts
• Updated use-inbox-data to use new endpoint instead of 3 useEntityRecords calls

Performance Impact

• Reduces from 3 REST requests to 1
• Single DB query vs 3 separate queries
• 30s cache reduces server load

Related

Main PR: #45339

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[jp-launch-control]

Code Coverage Summary

Cannot generate coverage summary while tests are failing. 🤐

Please fix the tests, or re-run the Code coverage job if it was something being flaky.

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