Hydrogen embrittlement can significantly accelerate fatigue crack initiation and growth in structural steels, posing a critical challenge for the integrity of components operating in hydrogen environments. Phase-field approaches provide a robust variational framework for modeling complex fatigue-driven fracture processes without explicit crack tracking; however, predictive simulation of hydrogen-assisted fatigue remains challenging due to the strong coupling between cyclic damage accumulation, hydrogen transport, microstructural effects, and computational efficiency in high-cycle regimes. This contribution presents a unified multiphysics phase-field framework for hydrogen-assisted fatigue fracture in metallic materials, integrating fatigue-driven toughness degradation, stress-assisted hydrogen diffusion, and hydrogen-dependent fracture resistance within a single energetic formulation. Fatigue damage accumulation is governed by an energetic degradation law acting on the fracture toughness, while hydrogen embrittlement is incorporated through a concentration-dependent reduction based on adsorption--decohesion concepts. To enable computationally efficient simulation of high-cycle fatigue while retaining mean-load and frequency effects, fatigue accumulation is performed in the cycle domain using an envelope-load strategy combined with a consistent cycle-to-time mapping. The framework is implemented within an open-source multi-application environment using a staggered solution scheme, adaptive mesh refinement, and an enhanced diffusivity treatment to represent rapid hydrogen transport along evolving crack surfaces. The methodology is demonstrated for a range of engineering-relevant applications, including hydrogen-assisted fatigue crack growth in homogeneous steels, rolling-induced anisotropy in pipeline materials, and heterogeneous welded joints comprising base metal, heat-affected zone, and weld metal. Validation against experimental compact tension and single-edge notched specimens shows good agreement in terms of crack paths, growth rates, orientation effects, and hydrogen--frequency interactions. The proposed framework provides a scalable and physically grounded tool for assessing hydrogen-assisted fatigue performance and supporting integrity assessment of metallic components in hydrogen service.