docs: añade documentación completa del proyecto

- README.md: descripción general, stack, instalación rápida
- docs/API.md: referencia completa de API REST y WebSocket
- docs/ARCHITECTURE.md: arquitectura del sistema con diagramas
- docs/INSTALLATION.md: guía detallada de instalación
- backend/.env.example: plantilla de configuración

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/.env.example

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+# ===========================================
+# Trivy Backend - Configuration
+# ===========================================
+
+# Database
+DATABASE_URL=postgresql+asyncpg://trivy:trivy@localhost:5432/trivy
+
+# Redis
+REDIS_URL=redis://localhost:6379
+
+# Anthropic API (Required for answer validation)
+ANTHROPIC_API_KEY=sk-ant-api03-YOUR_API_KEY_HERE
+
+# ===========================================
+# Game Settings
+# ===========================================
+
+# Penalty multiplier when failing a steal (0.5 = lose 50% of question points)
+STEAL_PENALTY_MULTIPLIER=0.5
+
+# Time multiplier for steal attempts (0.5 = half the original time)
+STEAL_TIME_MULTIPLIER=0.5
+
+# How long rooms stay in Redis (hours)
+ROOM_TTL_HOURS=3
+
+# ===========================================
+# Security
+# ===========================================
+
+# CORS origins (comma-separated)
+CORS_ORIGINS=http://localhost:3000,http://localhost:5173
+
+# Secret key for sessions (generate with: openssl rand -hex 32)
+SECRET_KEY=your-secret-key-here
+
+# ===========================================
+# Optional
+# ===========================================
+
+# Log level (DEBUG, INFO, WARNING, ERROR)
+LOG_LEVEL=INFO
+
+# Enable SQL echo (for debugging)
+SQL_ECHO=false