feat: AI Act Decision Tree — Zwei-Achsen-Klassifikation (GPAI + High-Risk)

Interaktiver 12-Fragen-Entscheidungsbaum für die AI Act Klassifikation
auf zwei Achsen: High-Risk (Anhang III, Q1-Q7) und GPAI (Art. 51-56, Q8-Q12).
Deterministische Auswertung ohne LLM.

Backend (Go):
- Neue Structs: GPAIClassification, DecisionTreeAnswer, DecisionTreeResult
- Decision Tree Engine mit BuildDecisionTreeDefinition() und EvaluateDecisionTree()
- Store-Methoden für CRUD der Ergebnisse
- API-Endpoints: GET/POST /decision-tree, GET/DELETE /decision-tree/results
- 12 Unit Tests (alle bestanden)

Frontend (Next.js):
- DecisionTreeWizard: Wizard-UI mit Ja/Nein-Fragen, Dual-Progress-Bar, Ergebnis-Ansicht
- AI Act Page refactored: Tabs (Übersicht | Entscheidungsbaum | Ergebnisse)
- Proxy-Route für decision-tree Endpoints

Migration 083: ai_act_decision_tree_results Tabelle

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

backend-compliance/migrations/083_ai_act_decision_tree.sql

@@ -0,0 +1,20 @@
+-- Migration 083: AI Act Decision Tree Results
+-- Stores results from the two-axis AI Act classification (High-Risk + GPAI)
+
+CREATE TABLE IF NOT EXISTS ai_act_decision_tree_results (
+    id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    tenant_id UUID NOT NULL,
+    project_id UUID,
+    system_name VARCHAR(500) NOT NULL,
+    system_description TEXT,
+    answers JSONB NOT NULL DEFAULT '{}',
+    high_risk_level VARCHAR(50) NOT NULL DEFAULT 'not_applicable',
+    gpai_result JSONB NOT NULL DEFAULT '{}',
+    combined_obligations JSONB DEFAULT '[]',
+    applicable_articles JSONB DEFAULT '[]',
+    created_at TIMESTAMPTZ DEFAULT NOW(),
+    updated_at TIMESTAMPTZ DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_ai_act_dt_tenant ON ai_act_decision_tree_results(tenant_id);
+CREATE INDEX IF NOT EXISTS idx_ai_act_dt_project ON ai_act_decision_tree_results(project_id) WHERE project_id IS NOT NULL;