Fix typo: Update 'diary_free' to 'dairy_free' in diet preferences

[coderabbitai]

Currently, both the backend and frontend use 'diary_free' as one of the diet preference values, which is a typo. This should be updated to 'dairy_free'.

Required changes:

1. Update the backend API to use 'dairy_free'
2. Update the frontend constant ENCOUNTER_DIET_PREFERENCE to use 'dairy_free'

Note: Both changes need to be coordinated to maintain compatibility.

References:

• Discussion: Encounter Qustions dropdown values moved to constants and i18 applied for all strings #9859 (comment)
• PR: Encounter Qustions dropdown values moved to constants and i18 applied for all strings #9859