Update a pronunciation item

Partial update - include only the fields you want to change. group_id cannot be changed via this endpoint. The final merged state must not combine spell_out: true with a non-empty replacement or ipa. An update that sets replacement or ipa must leave only one of them non-empty (send "" to clear the other); entries saved with both before 2026-10-09 keep working and accept updates that set neither. It must retain at least one effective option: nonblank replacement, valid nonblank IPA, spell_out enabled, or an explicit supported rate. Clearing the last effective option is rejected, including for inactive entries. Existing invalid entries remain readable and deletable but must be corrected before saving. Concurrent changes to the validated fields return 409; reload before retrying.