Add backend structure with Docker support and initial configuration files

Signed-off-by: Dmitrii <computer@yawaflua.tech>

Took 4 hours 25 minutes

backend/SpMega.Backend/Services/TokenService.cs

@@ -0,0 +1,40 @@
+using System.IdentityModel.Tokens.Jwt;
+using System.Security.Claims;
+using System.Text;
+using Microsoft.IdentityModel.Tokens;
+
+namespace SpMega.Backend.Services;
+
+public class TokenService(IConfiguration conf)
+{
+    private const int AccessTokenExpirationMinutes = 24;
+    private readonly string _issuer = conf["JWT__Issuer"] ?? "spmega.il.yawaflua.tech";
+    private readonly string _secret = conf["JWT__Secret"] ?? throw new InvalidOperationException("JWT__Secret is not configured.");
+
+    public string GenerateAccessToken(string userName, Guid uuid)
+    {
+        var claims = new List<Claim>
+        {
+            new(JwtRegisteredClaimNames.Name, userName),
+            new (JwtRegisteredClaimNames.UniqueName, uuid.ToString()),
+            new(JwtRegisteredClaimNames.AuthTime, DateTimeOffset.UtcNow.ToUnixTimeSeconds().ToString()),
+            new(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString())
+        };
+
+        var tokenHandler = new JwtSecurityTokenHandler();
+        var key = Encoding.ASCII.GetBytes(_secret);
+
+        var tokenDescriptor = new SecurityTokenDescriptor
+        {
+            Subject = new ClaimsIdentity(claims),
+            Expires = DateTime.UtcNow.AddHours(AccessTokenExpirationMinutes),
+            Issuer = _issuer,
+            SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(key), SecurityAlgorithms.HmacSha256Signature),
+            IssuedAt = DateTime.UtcNow
+        };
+
+        var token = tokenHandler.CreateToken(tokenDescriptor);
+
+        return tokenHandler.WriteToken(token);
+    }
+}