Prove board detail reads use split queries

[Chris0Jeky]

Summary\n\n- Add a real-SQLite regression for BoardRepository.GetByIdWithDetailsAsync with two columns, two cards, two labels, and card-label relationships.\n- Clear tracking and captured setup commands before the read, then assert the complete graph and multiple executed SELECT commands.\n- Keep production code unchanged.\n\n## Verification\n\n- dotnet test backend/tests/Taskdeck.Api.Tests/Taskdeck.Api.Tests.csproj -c Release -m:1 --filter FullyQualifiedName~BoardRepositoryIntegrationTests — 19/19\n- Mutation control: temporarily removed only BoardRepository.AsSplitQuery(); the new test failed with one combined SELECT, then the line was restored and the test passed.\n- git diff --check origin/main...HEAD — passed\n\n## Docs\n\nNo canonical docs changed; this is a bounded regression-proof slice.\n\n## Scope\n\nRefs #1133

[chatgpt-codex-connector]

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