The safe use of existing pipeline infrastructure for hydrogen transport is limited by hydrogen embrittlement and hydrogen-assisted fatigue, which are particularly critical in welded joints. This work presents a unified multiphysics framework that couples phase-field fracture with stress-assisted hydrogen diffusion to predict fatigue crack nucleation and growth under gaseous hydrogen environments in the high-cycle regime. Hydrogen transport is modeled by diffusion driven by concentration gradients and hydrostatic stress, and its mechanical effect is introduced through a hydrogen-dependent degradation of crack-growth resistance within the phase-field formulation. To enable efficient high-cycle simulations, an envelope-load strategy and adaptive cycle jumping are adopted, and adaptive mesh refinement is used to maintain resolution near evolving crack fronts and sharp concentration gradients. The framework is implemented in a finite-element multiphysics setting and validated on standard fracture and fatigue benchmarks before being applied to welded X65 pipeline steel, where base metal, heat-affected zone, and weld metal are explicitly distinguished. The simulations reproduce key trends of hydrogen-accelerated crack growth and reveal preferential crack propagation associated with local weld heterogeneity, with the heat-affected zone showing the highest sensitivity. A parametric analysis further clarifies the roles of hydrogen exposure and loading frequency, providing quantitative insights for integrity assessment and design of welded components intended for hydrogen service.