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/features/profile/profile_service.dart

@@ -12,7 +12,7 @@ class UpdateProfileRequest {
   final String? name;
   final int? heightCm;
   final double? weightKg;
-  final int? age;
+  final String? dateOfBirth;
   final String? gender;
   final String? activity;
   final String? goal;
@@ -22,7 +22,7 @@ class UpdateProfileRequest {
     this.name,
     this.heightCm,
     this.weightKg,
-    this.age,
+    this.dateOfBirth,
     this.gender,
     this.activity,
     this.goal,
@@ -34,7 +34,7 @@ class UpdateProfileRequest {
     if (name != null) map['name'] = name;
     if (heightCm != null) map['height_cm'] = heightCm;
     if (weightKg != null) map['weight_kg'] = weightKg;
-    if (age != null) map['age'] = age;
+    if (dateOfBirth != null) map['date_of_birth'] = dateOfBirth;
     if (gender != null) map['gender'] = gender;
     if (activity != null) map['activity'] = activity;
     if (goal != null) map['goal'] = goal;