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.dart

@@ -13,7 +13,8 @@ class User {
   final int? heightCm;
   @JsonKey(name: 'weight_kg')
   final double? weightKg;
-  final int? age;
+  @JsonKey(name: 'date_of_birth')
+  final String? dateOfBirth;
   final String? gender;
   final String? activity;
   final String? goal;
@@ -30,7 +31,7 @@ class User {
     this.avatarUrl,
     this.heightCm,
     this.weightKg,
-    this.age,
+    this.dateOfBirth,
     this.gender,
     this.activity,
     this.goal,
@@ -42,6 +43,19 @@ class User {
   factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json);
   Map<String, dynamic> toJson() => _$UserToJson(this);
 
+  int? get age {
+    if (dateOfBirth == null) return null;
+    final dob = DateTime.tryParse(dateOfBirth!);
+    if (dob == null) return null;
+    final now = DateTime.now();
+    int years = now.year - dob.year;
+    if (now.month < dob.month ||
+        (now.month == dob.month && now.day < dob.day)) {
+      years--;
+    }
+    return years;
+  }
+
   bool get hasCompletedOnboarding =>
-      heightCm != null && weightKg != null && age != null && gender != null;
+      heightCm != null && weightKg != null && dateOfBirth != null && gender != null;
 }

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,