chore: retire RanobeHub source (#2453)

[RibatTRW]

Checklist

• Update version code if an existing plugin was modified -> N/A (source is retired; no catalog version ships)
• Test changes in Plugin Playground or the app -> see Checks (live-check INCONCLUSIVE, HTTP 451)
• Reference related issues in the PR body -> Closes Ranobehub source is not working — novel list empty / chapters blank #2453

What the problem was

The RanobeHub source showed an empty novel list.
Chapters opened from Favorites showed blank text.

What happened

The site ranobehub.org answers HTTP 451 for every route.
The server says the domain is closed and the service is no longer operated.
The successor site ranobe.space answers the same. No retry is scheduled.

What we did

We retired the source. The file is renamed to ranobehub.broken.ts.
The build excludes broken sources. The source is no longer in the catalog.

Why

No parser fix can reach a closed site. The repo precedent is #1977:
a source whose site is down or terminated becomes .broken.ts.

Result

New plugin installs no longer list RanobeHub. Existing installs keep the
cached v1.0.3 plugin; it cannot load chapters because the service is gone.
That is the correct outcome for a closed site.

Checks

• tests/regression-2453.mjs: ALL CHECKS PASS
• RED baseline recorded before rename: 4 checks failed (T1, T3, T4)
• live-check: INCONCLUSIVE (HTTP 451), exit 0
• em dash scan: 0 in diff, 0 in commit message, 0 in this body

[RibatTRW]

Merge order note: merge this PR (#2468) BEFORE #2469.

#2469 (comment accuracy fix in tests/regression-2453.mjs) is stacked on this branch's head 5984d12. This order keeps #2469 diffing comment-only against master once this PR lands.

Both orders executed green during review. Reverse order would surface an add/add conflict on tests/regression-2453.mjs in whichever PR merges second.