chore: bump Ghost to 6.50.0; 6.49.0:0 → 6.50.0:0

[helix-nine]

Summary

Monitor-cycle upstream bump: Ghost 6.49.0 → 6.50.0 (startos/versions/current.ts → 6.50.0:0, manifest dockerTag → ghost:6.50.0-alpine).

MySQL stays pinned at 8.4.9 (per UPDATING.md, only bump MySQL when intentionally moving it). @start9labs/start-sdk is already at the latest published 1.5.3 — no SDK bump. npm update floated one transitive dep (fast-xml-builder 1.2.0 → 1.2.1).

Upstream highlights (6.50.0)

• ✨ Publisher gift links
• ✨ Configurable admin session max age
• ✨ System appearance (light/dark) mode in Ghost Admin
• 🐛 Fixed a stored XSS vulnerability + mangled structured data in JSON-LD output
• 🎨 Analytics and dark-mode improvements across the admin

Full notes: https://github.com/TryGhost/Ghost/releases/tag/v6.50.0

Minor bump, no breaking changes for the package; no state migration required (migrations.up remains a no-op).

Test plan

• npm run check (tsc --noEmit) — green
• make / s9pk pack — deferred to review per monitor-cycle policy

[helix-nine]

Thanks for the review, @MattDHill. Merged — Ghost 6.50.0:0 is out.