classifyTier() already honored a DB-backed providerOverrides list keyed
by an arbitrary provider-id string (built-in or custom), but nothing
exposed it through the UI or API. Adds GET/PUT /api/settings/tier-config,
a generic Advanced Settings tier selector wired into EditConnectionModal,
and makes TierCoverageWidget consult the same override before falling
back to registry-membership classification.
Owner decision: scope is the 3 real ProviderTier machine values
(free/cheap/premium) — the enum is not extended to 4.