feat: grant "eForm users" full planning view when no manager exists

[renemadsen]

Summary

• Adds a narrow fallback to TimePlanningPlanningService.Index: when no AssignedSite has IsManager=true system-wide, users in the "eForm users" security group are promoted to the admin-for-visibility path so the planning dashboard isn't empty in that degenerate state.
• Composite "Kun tid + WebAccess" users (auto-added to both groups by BackendConfigurationAssignmentWorkerServiceHelper) are explicitly excluded and stay restricted to their own site — honoring the rule that "kun tid" users must only see their own time.
• The 2026-04-10 filter (isAdmin = eFormAdminsGroup) is preserved; the fallback only fires when that gate fails.
• Scope is strictly TimePlanningPlanningService.Index. TimeSettingService.GetAvailableSites and AbsenceRequestService.GetInboxAsync are untouched.

Test plan

• In a tenant with zero AssignedSite.IsManager=true rows, log in as a non-admin user in "eForm users" → Time Planning > Plannings shows all workers.
• Flip one AssignedSite.IsManager=true (via property-worker UI) → same user now sees only their own site.
• Log in as a "Kun tid" user (property worker with timeRegistrationEnabled=true) in the same zero-manager tenant → still restricted to own site.
• Log in as a composite "Kun tid + WebAccess" user in the zero-manager tenant → still restricted to own site.
• Existing admin ("eForm admins" / admin role) flow unchanged — sees all workers regardless.

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