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>

backend/internal/user/calories_test.go

@@ -2,10 +2,48 @@ package user
 
 import (
 	"testing"
+	"time"
 )
 
 func ptr[T any](v T) *T { return &v }
 
+func TestAgeFromDOB_Nil(t *testing.T) {
+	if AgeFromDOB(nil) != nil {
+		t.Fatal("expected nil for nil input")
+	}
+}
+
+func TestAgeFromDOB_InvalidFormat(t *testing.T) {
+	s := "not-a-date"
+	if AgeFromDOB(&s) != nil {
+		t.Fatal("expected nil for invalid format")
+	}
+}
+
+func TestAgeFromDOB_ExactAge(t *testing.T) {
+	dob := time.Now().AddDate(-30, 0, 0).Format("2006-01-02")
+	age := AgeFromDOB(&dob)
+	if age == nil {
+		t.Fatal("expected non-nil result")
+	}
+	if *age != 30 {
+		t.Errorf("expected 30, got %d", *age)
+	}
+}
+
+func TestAgeFromDOB_BeforeBirthday(t *testing.T) {
+	// Birthday is one day in the future relative to today-25y, so age should be 24
+	now := time.Now()
+	dob := time.Date(now.Year()-25, now.Month(), now.Day()+1, 0, 0, 0, 0, time.UTC).Format("2006-01-02")
+	age := AgeFromDOB(&dob)
+	if age == nil {
+		t.Fatal("expected non-nil result")
+	}
+	if *age != 24 {
+		t.Errorf("expected 24, got %d", *age)
+	}
+}
+
 func TestCalculateDailyCalories_MaleMaintain(t *testing.T) {
 	cal := CalculateDailyCalories(ptr(180), ptr(80.0), ptr(30), ptr("male"), ptr("moderate"), ptr("maintain"))
 	if cal == nil {