INDEX_OPTIMIZE was dropped from the TaskType enum but still referenced
in _RUNNABLE_TASKS (views.py) and the frontend system-status-dialog,
causing an AttributeError at import time. Restore the enum value in the
model and migration so the serializer accepts it, but remove it from
_RUNNABLE_TASKS since index_optimize is a Tantivy no-op. Remove the
frontend "Run Task" button for index optimization accordingly.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>