fix: pass weight_unit to PR check endpoints

feat: update eval profiles for native unit field names (Task 29)

fix: update all test files for native unit field names

feat: update remaining routers for native unit system

feat: add unit system instructions to all AI prompts

feat: update AI router to use native unit fields in formatting

feat: update export service for native unit field names

feat: make deload service unit-aware

feat: make PR detection service unit-aware

feat: create unit_config and make warmup service unit-aware

feat: rename SetTarget target_weight_kg to target_weight

feat: rename Measurement value_cm to value for unit-agnostic storage

feat: rename User height_cm/weight_kg to height/weight, add unit fields

fix: add user-based PR check endpoint, round PR detail weights

fix: AI chat weight field name, create CycleTracker on plan create