feat(inventaris): stok keluar/retur/opname + detail unit, dan edit slip payroll (port upstream)
- inventoryService: port penuh dari upstream (issueStock FIFO/SN, returnStock, adjustStock opname, serial tersedia, statistik, detail barang) + kompatibilitas fork
- schema: kolom price/supplier (stock) dan remaining_stock/recipient/reference_no (log) via migrasi aman
- admin: route stok keluar/retur/opname + API detail & serial; modal + tombol aksi di halaman inventaris; viewStock yang tadinya undefined
- payroll: updateSlip() + route /payroll/slip/:id/update