Add photo upload as a real onboarding step

Photos are now step 2 of onboarding (right after basic info) instead
of only being manageable from the profile page later. Uploading still
goes through the existing /profile/photos endpoint; the new
POST /onboarding/photos just confirms at least one photo exists and
advances the step counter. Renumbered the OnboardingStep enum
accordingly (9 steps total now).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/models/user.go

@@ -26,6 +26,7 @@ type OnboardingStep int
 
 const (
 	StepBasicInfo OnboardingStep = iota + 1
+	StepPhotos
 	StepBodyInfo
 	StepProfileQuestions
 	StepEducation
@@ -58,6 +59,7 @@ var OnboardingStepOrder = []struct {
 	Key  string
 }{
 	{StepBasicInfo, "basic_info"},
+	{StepPhotos, "photos"},
 	{StepBodyInfo, "body_info"},
 	{StepProfileQuestions, "profile_questions"},
 	{StepEducation, "education"},