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/repository_integration_test.go

@@ -100,17 +100,17 @@ func TestRepository_Update_MultipleFields(t *testing.T) {
 	created, _ := repo.UpsertByFirebaseUID(ctx, "fb-multi", "multi@example.com", "Multi User", "")
 	height := 175
 	weight := 70.5
-	age := 25
+	dob := "2001-03-09"
 	gender := "male"
 	activity := "moderate"
 	goal := "maintain"
 	u, err := repo.Update(ctx, created.ID, UpdateProfileRequest{
-		HeightCM: &height,
-		WeightKG: &weight,
-		Age:      &age,
-		Gender:   &gender,
-		Activity: &activity,
-		Goal:     &goal,
+		HeightCM:    &height,
+		WeightKG:    &weight,
+		DateOfBirth: &dob,
+		Gender:      &gender,
+		Activity:    &activity,
+		Goal:        &goal,
 	})
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)