feat[BE]: Add requested_qty field to LayingTransferSource and update related logic for transfer operations

internal/database/migrations/20260119055524_add_requested_qty_to_laying_transfer_sources.down.sql

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+-- Rollback: Remove requested_qty column from laying_transfer_sources table
+
+ALTER TABLE laying_transfer_sources
+DROP COLUMN IF EXISTS requested_qty;

internal/database/migrations/20260119055524_add_requested_qty_to_laying_transfer_sources.up.sql

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+-- Add requested_qty column to laying_transfer_sources table
+-- This field stores the quantity requested by user during create/update
+-- Separate from UsageQty (FIFO consumed) and PendingUsageQty (FIFO pending)
+
+ALTER TABLE laying_transfer_sources
+ADD COLUMN requested_qty NUMERIC(15,3) DEFAULT 0 NOT NULL;
+
+-- Add comment for documentation
+COMMENT ON COLUMN laying_transfer_sources.requested_qty IS 'Quantity requested by user during create/update';