feat: Variantenmanagement — Sub-Projekte mit GAP-Analyse

Backend:
- parent_project_id auf iace_projects (DB + Go Struct)
- POST/GET /variants + GET /variant-gap Endpoints
- GAP-Analyse: Differenz Hazards/Massnahmen/Kategorien

Frontend:
- VariantPanel auf Projekt-Uebersicht
- Variante erstellen Dialog
- Sidebar-Anzeige (Variantenanzahl / Basis-Link)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ai-compliance-sdk/internal/iace/models_api.go

@@ -12,6 +12,7 @@ import (
 
 // CreateProjectRequest is the API request for creating a new IACE project
 type CreateProjectRequest struct {
+	ParentProjectID *uuid.UUID      `json:"parent_project_id,omitempty"`
 	MachineName     string          `json:"machine_name" binding:"required"`
 	MachineType     string          `json:"machine_type" binding:"required"`
 	Manufacturer    string          `json:"manufacturer" binding:"required"`
@@ -199,6 +200,28 @@ type ValidateMitigationHierarchyResponse struct {
 	Warnings []string `json:"warnings,omitempty"`
 }
 
+// VariantGap compares a variant project against its base project
+type VariantGap struct {
+	BaseProject ProjectSummary `json:"base_project"`
+	Variant     ProjectSummary `json:"variant"`
+	Gap         GapDetail      `json:"gap"`
+}
+
+// ProjectSummary is a lightweight project view for gap comparisons
+type ProjectSummary struct {
+	ID           uuid.UUID `json:"id"`
+	Name         string    `json:"name"`
+	HazardCount  int       `json:"hazard_count"`
+	MeasureCount int       `json:"measure_count"`
+}
+
+// GapDetail describes the delta between variant and base project
+type GapDetail struct {
+	AdditionalHazards  int      `json:"additional_hazards"`
+	AdditionalMeasures int      `json:"additional_measures"`
+	CategoriesAffected []string `json:"categories_affected"`
+}
+
 // CompletenessGate represents a single gate in the project completeness checklist
 type CompletenessGate struct {
 	ID       string `json:"id"`