Store date_of_birth (DATE) instead of a static age integer so that age
is always computed dynamically from the stored date of birth.
- Migration 011: adds date_of_birth, backfills from age, drops age
- AgeFromDOB helper computes current age from YYYY-MM-DD string
- User model, repository SQL, and service validation updated
- Flutter: User.age becomes a computed getter; profile edit screen
uses a date picker bounded to [today-120y, today-10y]
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>