feat: UGC server that makes and serves player models' meshes and icons

A new server (dUgcServer, started by master with enable_ugc_server=1) that
takes unprocessed ugc and ugc_modular_build rows from the database and makes
what the client downloads with UGCUSE3DSERVICES: an optimized NIF (hidden
faces removed, ambient occlusion baked into vertex colors) and a 128px DDS
icon for player models, rendered by a software rasterizer from the client's
LDD brick primitives, and icons for cars and rockets assembled from their
modules per ModularBuildComponent/ModuleComponent. It serves them, with the
models' LXFML, over HTTP in the client's UGCC<dc>/3DOPTIMIZED and
IMAGE128DDS layout with .gz and .checksum files, and keeps its folder under
a size cap.

Processing state lives in ugc.is_optimized plus new processed_at,
process_attempts and process_error columns (and the same on
ugc_modular_build). ServiceType::UGC is appended. NifFile moves to dCommon
and records named node transforms for the modules' attach points.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dUgcServer/UgcFormats.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <string>
+#include <string_view>
+#include <vector>
+
+#include "UgcModel.h"
+#include "UgcRender.h"
+
+/**
+ * The files the UGC server writes: the mesh as a Gamebryo .nif the client loads, icons as .png and .dds, and the
+ * client's .checksum files for its downloads. Pure (data in, bytes out).
+ */
+namespace UgcFormats {
+	struct NifShape {
+		std::string name;
+		const UgcModel::Mesh* mesh{};
+		bool transparent{};
+	};
+
+	/**
+	 * A Gamebryo 20.3.0.9 file (user version 0, as the client's own meshes): a root NiNode named `rootName` with one
+	 * NiTriShape per shape (at most 65535 vertices and triangles each: UgcModel::Split), with vertex colors that are
+	 * the diffuse and ambient color (NiVertexColorProperty), a white NiMaterialProperty, and for transparent shapes an
+	 * NiAlphaProperty blending with the vertex alpha.
+	 */
+	std::string WriteNif(const std::string& rootName, const std::vector<NifShape>& shapes);
+
+	// PNG (8-bit RGBA)
+	std::string EncodePng(const UgcRender::Image& image);
+
+	// DDS, uncompressed 32-bit BGRA without mipmaps
+	std::string EncodeDds(const UgcRender::Image& image);
+
+	// Lowercase hex MD5 of `data`
+	std::string Md5Hex(std::string_view data);
+
+	// What the client reads from a UGC file's .checksum: the MD5 and size of the file as it is after inflating it
+	std::string ChecksumXml(std::string_view data);
+}