Phase 11: real block textures for 2D tiles

Layers texture-averaged colors on top of palette.rs's hand-picked table as a
fallback (color_for_textured), not a hard replacement — biome-tinted/animated
blocks (grass top, leaves, water, lava) deliberately keep the hand-picked
color since averaging their raw jar textures would be wrong, not just
imprecise.

Vanilla: worker/src/textures.rs downloads Mojang's official client jar
directly from launchermeta/piston-meta/piston-data (same endpoints the real
launcher uses, gated behind ACCEPT_MINECRAFT_EULA=true, off by default,
mirrors BlueMap's accept-download) and averages every block texture, cached
to disk so it's not re-downloaded every restart.

Modded: ingests block_registry/block_textures messages from the mod (new
Postgres tables + MinIO storage in api/src/textures.ts) — sent once per
connection over the existing WS gateway.

texturepacks/<name>/ (Dynmap-style flat PNGs) lets an operator override the
vanilla defaults worker-wide via TEXTURE_PACK. A per-server texturePack
admin column/API exists for the same purpose, but render-time per-server
resolution (of both the admin selection and the ingested modded textures) is
explicitly deferred — the worker still applies one process-wide palette; true
per-server resolution needs the render pipeline to thread a server-scoped
palette through the batch/GPU-dispatch path, judged too big a change for this
phase. Documented in README.

Docker was unavailable in this dev environment for the usual integration-test
verification; bunx tsc --noEmit was used as a fallback static check instead
(clean except 2 pre-existing unrelated errors in markers.test.ts).

frontend/src/public/js/admin.js

@@ -61,6 +61,9 @@ function adminpanel() {
           anonymousChatAllowed: server.anonymousChatAllowed,
           waypointFormat: server.waypointFormat,
           playerPositionsVisible: server.playerPositionsVisible,
+          // Empty string means "no override" — normalize to null so it matches the schema's
+          // nullable column and admin.ts's "null clears it" validation path.
+          texturePack: server.texturePack ? server.texturePack.trim() || null : null,
         }),
       });
       server.status = res.ok ? "saved" : "save failed";