Follow-up: seo.py display names from core.constants (after #9621/#9624 merge) #9636

Small dedup follow-up: #9621 introduced local _LIBRARY_NAMES/_LANGUAGE_NAMES maps in api/routers/seo.py (derived from the registry, but locally), and #9624 added the shared LIBRARY_NAMES/LANGUAGE_NAMES to core/constants.py.

Once both PRs are merged: replace seo.py's local derivations with imports from core.constants, so exactly one derived name map exists. Was deliberately not done cross-PR to avoid a merge conflict.

~5-line change + existing tests (tests/unit/api/test_seo_helpers.py::TestDisplayNameMaps keeps guarding coverage).