fix(ugc): store meshes compressed, serve LXFML from the database, log client downloads

Each model took about 18 MB (the .nif uncompressed beside its .gz, the
unbaked .nif for the dashboard), so the 2 GB cap held about 110 models and
the server kept evicting and remaking them while clients got 408s. Only the
.gz files are stored now (the dashboard's copies are inflated when asked
for), and the previous version keeps only its icon, stats and .nif.gz.

The LXFML needs no making: its downloads come from the ugc row (kept in
memory for the last few), so they're never waited for or evicted. Every
client download is logged with its answer.

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

dUgcServer/UgcStorage.h

@@ -57,6 +57,10 @@ public:
 	// Removes the least recently used items until the total is at most `maxBytes`; what was removed
 	std::vector<Entry> Evict(uint64_t maxBytes) const;
 
+	// An item's .nif (`name` model.nif, model.noao.nif or previous.model.nif): stored compressed (<name>.gz), or as it is
+	// (made by older versions); nullopt when there's neither
+	std::optional<std::string> ReadNif(Kind kind, LWOOBJID id, const std::string& name) const;
+
 	// Names the server writes (and serves); anything else is refused
 	static bool IsKnownFile(const std::string& name);