feat(ugc): answer the client's UGC manifest requests without 3D services

With UGCUSE3DSERVICES=7:0 (the client's default) the client asks its world for
a blueprint file's MD5 and size (REQUEST_UGC_MANIFEST_INFO, world 27) and then
downloads BrickModels/UserMade/<id % 1000>/<id>.<ext>.sd0. Layouts checked in
the 1.10.64 client: the request is a u64 blueprint id and a u8 resource type;
the answer (UGC_MANIFEST_RESPONSE, client 60) repeats them and adds a u8 valid,
the u32 size and the 16 byte MD5 of the inflated file, 37 bytes after the 0x53
exactly or the client drops it.

- dNet: WorldPackets::RequestUgcManifestInfo, ClientPackets::UgcManifestResponse
  (eUgcResourceType), with byte tests against the client's layouts.
- Database: ugc_file_checksums (per model or module combination and file) and
  ugc_modular_build.combination_id (migrations 89 / 72), GetUgcFileChecksum
  looks a blueprint up as a model, else as a build through its combination.
- UGC server: every download is also written as .sd0 (Sd0::Compress); workers
  hand the checksums back and the main thread stores them; old items get their
  sd0 icon and checksum, and builds their combination id, once at start-up, a
  few per tick; serves <dir>/BrickModels/UserMade/<bucket>/<id>.<ext>.sd0 under
  client_path, /<folder>/UserBrickModels and the root (.hkx 404).
- World: UgcManifest answers on the main thread with one indexed query per
  request; files not made yet are answered when they are (looked at again
  every 5 seconds), and a model in its quiet period is made right away. No
  worker threads, HTTP or file reads in the world.

Off by default (ugc_manifest=0): checked in game, the client then downloads
from http://127.0.0.1:80/lwoclient/UserBrickModels/ whatever its boot.cfg says
and logs the player out when it can't connect, so icons need the UGC server on
port 80 of each player's machine. docs/UgcServer.md has the details.

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

migrations/dlu/mysql/89_ugc_file_checksums.sql

@@ -0,0 +1,19 @@
+/* UGC server: ugc_file_checksums holds the MD5 (lowercase hex) and size of each file it made, as the game client has
+   it after inflating the download (kind 0: a player model, storage_id its ugc id; kind 1: a combination of car or
+   rocket modules, storage_id the combination's id). ugc_modular_build.combination_id is the combination a build was
+   made as (0 until the UGC server has seen it). Worlds read both to answer the client's REQUEST_UGC_MANIFEST_INFO.
+   See docs/UgcServer.md. */
+CREATE TABLE IF NOT EXISTS ugc_file_checksums (
+	kind INT NOT NULL,
+	storage_id BIGINT NOT NULL,
+	file VARCHAR(32) NOT NULL,
+	md5 CHAR(32) NOT NULL,
+	size INT UNSIGNED NOT NULL,
+	PRIMARY KEY (kind, storage_id, file)
+);
+
+SET @dlu_column = (SELECT IF(COUNT(*) = 0, 'ALTER TABLE ugc_modular_build ADD COLUMN combination_id BIGINT NOT NULL DEFAULT 0', 'DO 0') FROM information_schema.columns
+	WHERE table_schema = DATABASE() AND table_name = 'ugc_modular_build' AND column_name = 'combination_id');
+PREPARE dlu_column_stmt FROM @dlu_column;
+EXECUTE dlu_column_stmt;
+DEALLOCATE PREPARE dlu_column_stmt;

migrations/dlu/sqlite/72_ugc_file_checksums.sql

@@ -0,0 +1,11 @@
+/* UGC server: the checksums of the files it made and each car or rocket build's combination. See the MySQL migration. */
+CREATE TABLE IF NOT EXISTS ugc_file_checksums (
+	kind INTEGER NOT NULL,
+	storage_id BIGINT NOT NULL,
+	file TEXT NOT NULL,
+	md5 TEXT NOT NULL,
+	size INTEGER NOT NULL,
+	PRIMARY KEY (kind, storage_id, file)
+);
+
+ALTER TABLE ugc_modular_build ADD COLUMN combination_id BIGINT NOT NULL DEFAULT 0;