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.

.gitignore

@@ -0,0 +1,15 @@
+__pycache__/
+*.pyc
+*.pyo
+.venv/
+venv/
+.env
+*.egg-info/
+.pytest_cache/
+.coverage
+htmlcov/
+dist/
+build/
+node_modules/
+.DS_Store
+*.log