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;
 }