fix: keep Housekeeping plans alive during inventory

[IUnknown404I]

Summary

• give filesystem-intensive Housekeeping plans a dedicated bounded 30-minute Web request window instead of the ordinary 10-second timeout
• show deterministic loading states, suppress duplicate planners, keep execution fail-closed, and replace raw abort text with operator-safe errors
• serialize read-only plans with execution under the canonical Housekeeping lock so a disconnected client cannot start competing inventory work

Verification

• 198 focused/integrated backend Housekeeping and API tests passed
• all 228 Web tests passed
• production Web build passed
• independent focused lifecycle/destructive-authority review: PASS, no High or blocking Medium findings

No cleanup execution, production activation, tag, or release publication is part of this PR.