create Infrastructure directory

src/Core/Infrastructure/Infrastructure.Jwt/IJwtService.cs

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+namespace Service.Core.Jwt;
+
+public interface IJwtService
+{
+    string GenerateJwt(Guid userId, string username, DateTime expiry);
+}

src/Core/Infrastructure/Infrastructure.Jwt/Infrastructure.Jwt.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+   <PropertyGroup>
+      <TargetFramework>net10.0</TargetFramework>
+      <ImplicitUsings>enable</ImplicitUsings>
+      <Nullable>enable</Nullable>
+      <RootNamespace>Service.Core.Jwt</RootNamespace>
+   </PropertyGroup>
+
+   <ItemGroup>
+      <PackageReference Include="Microsoft.IdentityModel.JsonWebTokens" Version="8.2.1" />
+      <PackageReference Include="System.IdentityModel.Tokens.Jwt" Version="8.2.1" />
+   </ItemGroup>
+</Project>

src/Core/Infrastructure/Infrastructure.Jwt/JwtService.cs

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+using System.Security.Claims;
+using System.Text;
+using Microsoft.IdentityModel.JsonWebTokens;
+using Microsoft.IdentityModel.Tokens;
+using JwtRegisteredClaimNames = System.IdentityModel.Tokens.Jwt.JwtRegisteredClaimNames;
+
+namespace Service.Core.Jwt;
+public class JwtService : IJwtService
+{
+    private readonly string? _secret = Environment.GetEnvironmentVariable("JWT_SECRET");
+    public string GenerateJwt(Guid userId, string username, DateTime expiry)
+    {
+        var handler = new JsonWebTokenHandler();
+
+        var key = Encoding.UTF8.GetBytes(_secret ?? throw new InvalidOperationException("secret not set"));
+
+        // Base claims (always present)
+        var claims = new List<Claim>
+        {
+            new(JwtRegisteredClaimNames.Sub, userId.ToString()),
+            new(JwtRegisteredClaimNames.UniqueName, username),
+            new(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString())
+        };
+
+        var tokenDescriptor = new SecurityTokenDescriptor
+        {
+            Subject = new ClaimsIdentity(claims),
+            Expires = expiry,
+            SigningCredentials = new SigningCredentials(
+                new SymmetricSecurityKey(key),
+                SecurityAlgorithms.HmacSha256)
+        };
+
+        return handler.CreateToken(tokenDescriptor);
+    }
+}