~anthropic/claude-sonnet-latest → A (6:4)

Side A fixes a real, spreading bug: hardcoded hostnames (staging.sorter.social, InetAddress lookups) across six backend files that would break OAuth callbacks and redirects on custom hosts, consolidating them into a single reusable util.get-base-url with proper HOSTNAME/env fallback plus a matching frontend fix and CI staging trigger. Side B reworks an auth form to avoid a redirect via innerHTML morphing, which is a reasonable UX improvement but is narrower in scope, affects only one flow, and introduces added client-side complexity without fixing a correctness bug of comparable breadth.