feat: replace age integer with date_of_birth across backend and client

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>

client/lib/shared/models/user.g.dart

@@ -13,7 +13,7 @@ User _$UserFromJson(Map<String, dynamic> json) => User(
   avatarUrl: json['avatar_url'] as String?,
   heightCm: (json['height_cm'] as num?)?.toInt(),
   weightKg: (json['weight_kg'] as num?)?.toDouble(),
-  age: (json['age'] as num?)?.toInt(),
+  dateOfBirth: json['date_of_birth'] as String?,
   gender: json['gender'] as String?,
   activity: json['activity'] as String?,
   goal: json['goal'] as String?,
@@ -29,7 +29,7 @@ Map<String, dynamic> _$UserToJson(User instance) => <String, dynamic>{
   'avatar_url': instance.avatarUrl,
   'height_cm': instance.heightCm,
   'weight_kg': instance.weightKg,
-  'age': instance.age,
+  'date_of_birth': instance.dateOfBirth,
   'gender': instance.gender,
   'activity': instance.activity,
   'goal': instance.goal,