Restore backoffice guard and content paging fixes dropped in v17->v18 merge

[KevinJump]

Summary

An audit comparing v17/main against v18/main found that the squash-merge in #992 ("Merge v17/main into v18/main (2026-07-11)") successfully ported the large majority of v17's work (perf fixes, localization, blueprint path/parent resolution, image mapper split, clean-folder fix, all frontend changes) — but two fixes didn't survive the port:

• uSyncBackOfficeComposer.cs: the IsUmbracoBackOfficeEnabled() guard (originally added in v17 Support front end only umbraco config.  #962) was present but commented out, so uSync now registers itself unconditionally — including on headless/delivery-only servers where it shouldn't run. Restored the guard.
• ContentHandler.cs: during v18's refactor that centralized paged child-fetching into PublishableContentHandlerBase, ContentHandler lost the concrete-typed fetching it had in v17 (part of the perf work in v17 Perf: scope handler runtime-cache per operation (#5) + skip clean-up scan on flat+guid (#3) #990). The shared base now returns generic IEntitySlim for non-root items, which fails SyncHandlerRoot.ExportAllAsync's is TObject check and forces an extra contentService.GetById call per descendant item during export/import. MediaHandler was never affected since it already overrides GetChildItemsAsync directly with concrete IMedia — ContentHandler now mirrors that same pattern using _contentService.GetPagedChildren.

Everything else checked out as correctly ported; these are the only two gaps found.

Notes for reviewers

• Used the explicit 8-parameter GetPagedChildren overload (matching v17's original call) rather than the shorter 4-arg form MediaHandler uses for IMediaService, because IContentService's equivalent short overload is marked [Obsolete] in Umbraco 18.0.2 and would otherwise produce a CS0618 warning.
• No existing test exercises either code path directly (the composer guard or this specific paging call), so there's no new test coverage — these are behavioral/perf fixes, not bug fixes with a repro test.

Test plan

• dotnet build uSync.slnx — 0 warnings, 0 errors
• dotnet test uSync.Tests/uSync.Tests.csproj — 148/148 passing
• Manual/staging verification that a backoffice-disabled site no longer registers uSync, and that exporting a nested content tree no longer issues extra per-item GetById calls

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