feat: implement FraudShield MVP — backend API + frontend alerts

Backend (Python 3.12/FastAPI):
- 5 models: Transaction, DetectionRule, FraudAlert, AuditLog, RuleSnapshot
- Rule engine: 4 rule types (amount, location, time, pattern)
- 11 API routes: health, auth, transactions, alerts
- Sync ingestion pipeline with explanation generation
- JWT auth with 3 roles (analyst, admin, senior)
- Seed script with 5 default rules + 50 synthetic transactions

Frontend (React 19/TypeScript/Tailwind):
- Auth: login page, JWT token management, role-based routing
- Alerts: queue with filters, detail panel, decision workflow
- Layout: responsive sidebar, top bar, protected routes
- Full TypeScript, zero ts-ignore, Tailwind CSS

Build: Backend imports clean, Frontend tsc --noEmit passes,
vite build produces 296KB production bundle.

frontend/vite.config.ts

@@ -0,0 +1,11 @@
+import { defineConfig } from 'vite';
+import react from '@vitejs/plugin-react';
+
+export default defineConfig({
+  plugins: [react()],
+  server: {
+    proxy: {
+      '/api': 'http://localhost:8000',
+    },
+  },
+});