#124 Fastify v4 to v5 migration

Summary

• Fastify v5 migration complete: deps bumped, Node engine set to 20+, redirect signature fixed, HEAD ordering corrected.
• URL/host handling normalized via getRequestHost() and applied across handlers, auth, notifications, ActivityPub, and server headers to fix port/subdomain consistency.
• WebSocket notifications fixed for v5: correct handler signature, async WebID resolution, Updates‑Via includes port; all notifications tests pass.
• Live reload: watcher now closes on shutdown; tests use OS temp dir for Windows; live‑reload test now exits cleanly.
• README Node.js requirement updated to 20+.
• Full test run: npm test passing (224/224).

Notes

• Live reload test previously hung because the filesystem watcher stayed open; now closed via Fastify onClose.
• README prereqs updated to Node.js 20+ to align with engines.