B2B e-invoicing in 2026: XRechnung vs. ZUGFeRD and how to implement it technically

B2B mandatory since 01.01.2025: why PDF alone isn't enough, the difference between XRechnung and ZUGFeRD, KoSIT validation in the send path, GoBD-compliant archive with hash lock in Supabase Storage, and a receive workflow via email inbox.