chore: bump Ghost to 6.55.0; 6.54.1:0 → 6.55.0:0

[helix-nine]

Bumps Ghost from 6.54.1 to 6.55.0 (ghost:6.55.0-alpine, verified on Docker Hub for amd64 + arm64).

Upstream

6.55.0 is a bug-fix release. Highlights relevant to self-hosters:

• Members were unsubscribed from every newsletter when unsubscribing from one.
• Broken admin views for Contributor and Author staff users.
• Redirect capture groups ($1) were not substituted inside query strings.
• Duplicate reply-notification emails, broken internal links in automation emails, and feature image alt text dropped when too long.

The one feature in the release (configurable sidebar trial banner via hostSettings) is Ghost(Pro)-only and has no effect here.

Full changelog: TryGhost/Ghost@v6.54.1...v6.55.0

Not changed

• MySQL stays pinned at mysql:8.4.10. UPDATING.md says to move it only deliberately; 8.4.11 is available if the team wants the MySQL patch to ride along, but nothing in this Ghost release requires it.
• @start9labs/start-sdk is already at 2.0.9, the latest published version — no SDK bump, no lockfile churn.
• No package dependencies, so nothing to refresh there.

Test plan

• npm run check — green.
• Version bumped to 6.55.0:0 in startos/versions/current.ts, release notes written in all five locales. Registry's latest published version is 6.53.0:2, so 6.55.0:0 is free.
• No user-visible package behavior changed, so README.md and instructions.md need no updates.