refactor: Remove Base64String value object per YAGNI principle

Remove unnecessary Base64String value object:

  - Performance overhead (validation runs twice: value object + FluentValidation)

  - Unnecessary abstraction (Convert.FromBase64String already validates)

  - YAGNI principle: use string directly + extension methods if needed

Replaced with:

  - Direct string usage in DTOs

  - FluentValidation for Base64 format validation

  - FormatException handling in ExceptionHandlingMiddleware (maps to 400)

Result: Simpler code, better performance, same validation coverage

DocumentOperator.Domain/Models/ValueObjects/Base64String.cs

@@ -1,59 +0,0 @@
-namespace DocumentOperator.Domain.Models.ValueObjects;
-
-public sealed class Base64String
-{
-    public string Value { get; }
-
-    private Base64String(string value)
-    {
-        Value = value;
-    }
-
-    public static Base64String Create(string value)
-    {
-        if (string.IsNullOrWhiteSpace(value))
-            throw new Common.Exceptions.DomainValidationException("Base64 string cannot be empty.");
-
-        // Validierung: Ist es gültiges Base64?
-        try
-        {
-            Convert.FromBase64String(value);
-        }
-        catch (FormatException)
-        {
-            throw new Common.Exceptions.DomainValidationException("Invalid Base64 format.");
-        }
-
-        return new Base64String(value);
-    }
-
-    public static Base64String FromByteArray(byte[] bytes)
-    {
-        if (bytes == null || bytes.Length == 0)
-            throw new Common.Exceptions.DomainValidationException("Byte array cannot be null or empty.");
-
-        var base64 = Convert.ToBase64String(bytes);
-        return new Base64String(base64);
-    }
-
-    public byte[] ToByteArray()
-    {
-        return Convert.FromBase64String(Value);
-    }
-
-    public override string ToString() => Value;
-
-    // Equality (wichtig für Value Objects!)
-    public override bool Equals(object? obj)
-    {
-        if (obj is not Base64String other)
-            return false;
-
-        return Value == other.Value;
-    }
-
-    public override int GetHashCode()
-    {
-        return Value.GetHashCode();
-    }
-}