feat(fleet): per-product budgets with pause/resume (Phase 3 Slice 3)
- FleetBudgetDoc: ceilingUsd, window, spentUsd, status (active/paused)
- FLEET_BUDGETS flag (default OFF = no enforcement, unchanged behavior)
- Enforcement in claimNextJob: paused or ceiling-exceeded → null
- accrueSpend(): monotonic spend accumulation, auto-pause at ceiling
- Budget routes: GET/PUT /fleet/budgets/:productId, pause, resume
- UpsertBudgetSchema for route validation
- 7 new coordinator tests (ceiling, auto-pause, manual pause/resume,
flag OFF bypass, monotonic accounting, cross-product isolation)
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
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