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).

api/drizzle/0005_texture_pack.sql

@@ -0,0 +1 @@
+ALTER TABLE "servers" ADD COLUMN IF NOT EXISTS "texture_pack" text;

api/drizzle/0006_block_textures.sql

@@ -0,0 +1,14 @@
+CREATE TABLE IF NOT EXISTS "block_registry" (
+	"server_id" uuid NOT NULL REFERENCES "servers"("id") ON DELETE CASCADE,
+	"block_id" integer NOT NULL,
+	"name" text NOT NULL,
+	PRIMARY KEY ("server_id", "block_id")
+);
+
+CREATE TABLE IF NOT EXISTS "block_textures" (
+	"server_id" uuid NOT NULL REFERENCES "servers"("id") ON DELETE CASCADE,
+	"name" text NOT NULL,
+	"storage_key" text NOT NULL,
+	"updated_at" timestamptz NOT NULL DEFAULT now(),
+	PRIMARY KEY ("server_id", "name")
+);