load_registries + load_mirrors normally only ADD missing defaults to
the persisted JSON — explicit removals stick. After retiring the .23
Hetzner VPS we need the opposite: existing nodes have .23 baked into
their saved configs and would spend seconds per install/update timing
out against a dead host until the operator manually removes it via
the Settings UI.
Add a targeted one-time migration in both loaders: if any saved entry
has 23.182.128.160 in its URL, drop it on load and rewrite the file.
This is an exception to the usual "explicit removals stick" rule —
the user never chose to add this mirror, it was a default.
Narrow-scope migration (one hardcoded IP match, no schema version)
because the cost/benefit of a general migration system isn't worth
it for a single decommissioned host. Future retirements can follow
the same pattern.
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