feat(assets): creature artwork from the shard's own client (Phase 3)

Until now the only way a creature got a picture on this site was for an
operator to open UOFiddler on a desktop, export sprites by hand, copy them to
the web host and write a spawnAtlas.art.json naming each one. Almost nobody
did, so shard_spawn_creatures.art was NULL on every install.

The shard has had those files the whole time. Admin -> Shard -> Import now
walks its asset manifest, fetches only the sprites whose hash changed, writes
them under uploads/atlas/, asks the shard for a body id per atlas creature
(§8: it CONSTRUCTS the creature and reads Body.BodyID, which is the only thing
that is right for a shard's own custom creatures) and points each creature at
its picture. On a stock client that is 787 portraits, about a megabyte.

**The one thing v8.md §12 got wrong, and it is not cosmetic.** It says
`shard_spawn_creatures.art` "starts being filled by the import". That table is
emptied and refilled by replaceAtlas on EVERY atlas refresh, and a refresh runs
on every boot -- so a filename stored there would be destroyed by an ordinary
re-parse of the ServUO tree, with the next Update finding the client files
unchanged, reporting "nothing to do", and never restoring it. Nothing would
report a fault; the pictures would just be gone.

So the assets and the body map live in their own tables outside that blast
radius, and applyAtlas re-derives `art` on the way past as
`{ ...derived, ...operatorMap }` -- which is also the one place "the operator's
own artwork wins" is enforced, on every rebuild rather than only at import.

Smaller decisions worth not rediscovering:

- The derivation joins on the catalogue KEY, not on the body id. The simpler
  join is correct today and stops being correct the moment phase 6 adds
  body/400/a2/f0, at which point one slug matches dozens of rows.
- Filenames are content-addressed. A stable name overwritten in place leaves
  every browser and CDN serving the previous client's sprite, with the database
  row perfectly correct.
- An unchanged key whose FILE is missing is fetched again. The row and the disk
  can disagree (a wiped uploads volume, a restore from a dump), and a broken
  image on a creature page is worse than one re-fetched sprite.
- A key the shard cannot render is not a failure. Two thirds of the playable
  ghost and gargoyle bodies have no art on a stock client, and an import that
  reported eight failures every time would teach an operator to ignore the panel.
- A key that VANISHED from the manifest needs review before anything changes:
  an unmounted client volume and a deliberate downgrade look identical here.

23 new tests; 674 server and 42 client tests pass. The SQL was also run against
a real MariaDB, which is what proved the CONCAT join and the singleton CHECK.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016wDDVXWMDz82WqE1i969r4

routes.manifest.json

@@ -16,6 +16,11 @@
       "path": "/api/v1/admin/shard/accounts",
       "tier": "public"
     },
+    {
+      "method": "GET",
+      "path": "/api/v1/admin/shard/assets",
+      "tier": "public"
+    },
     {
       "method": "GET",
       "path": "/api/v1/admin/shard/atlas",
@@ -271,6 +276,11 @@
       "path": "/api/v1/admin/shard/account",
       "tier": "public"
     },
+    {
+      "method": "POST",
+      "path": "/api/v1/admin/shard/assets/import",
+      "tier": "public"
+    },
     {
       "method": "POST",
       "path": "/api/v1/admin/shard/atlas/approve",