feat: Sistema de Usuarios - Backend CRUD + JWT Auth (Issue #1)

Implementación fundacional del proyecto PruebaGentle:
- Arquitectura Clean/Hexagonal: Core, Infrastructure, API
- 6 Stored Procedures para CRUD + autenticación
- JWT authentication con BCrypt password hashing
- Docker Compose (SQL Server + Backend)
- Solución .NET 10 con Dapper + SqlClient

Closes #1

docker-compose.yml

@@ -0,0 +1,38 @@
+version: '3.8'
+
+services:
+  sqlserver:
+    image: mcr.microsoft.com/mssql/server:2022-latest
+    environment:
+      ACCEPT_EULA: "Y"
+      SA_PASSWORD: "YourStrong@Password123"
+      MSSQL_PID: "Developer"
+    ports:
+      - "1433:1433"
+    volumes:
+      - sqlserver_data:/var/opt/mssql
+      - ./Backend/Sql:/docker-entrypoint-initdb.d
+    healthcheck:
+      test: /opt/mssql-tools/bin/sqlcmd -S localhost -U sa -P "YourStrong@Password123" -Q "SELECT 1" || exit 1
+      interval: 10s
+      timeout: 3s
+      retries: 10
+      start_period: 30s
+
+  backend:
+    build:
+      context: .
+      dockerfile: Backend/Dockerfile
+    ports:
+      - "5000:8080"
+    environment:
+      - ASPNETCORE_ENVIRONMENT=Development
+      - ConnectionStrings__DefaultConnection=Server=sqlserver;Database=PruebaGentle;User Id=sa;Password=YourStrong@Password123;TrustServerCertificate=true;
+      - JwtSettings__Secret=ThisIsA32CharacterLongSecretKey!!
+      - JwtSettings__ExpirationHours=24
+    depends_on:
+      sqlserver:
+        condition: service_healthy
+
+volumes:
+  sqlserver_data: