Adds @ConditionalOnProperty(coexpression_enabled=true) to the legacy
CoExpressionController so /api/molecular-profiles/co-expressions/fetch
is disabled by default. The column-store implementation remains
always-on. Existing controller test sets the property via
@TestPropertySource so it continues to wire up the bean.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>