Phase 13: real non-cube block models via blockstate/model JSON resolution

Adds worker/src/models.rs (parent-chain blockstate/model resolution,
texture-variable substitution reusing Phase 12's atlas keys), routes
non-cube blocks through new per-element mesh emission in mesh.rs while
leaving full-cube blocks on the existing cube mesher, and threads a
per-server ModelContext (vanilla worker-wide + modded per-job) through
main.rs. Modded model JSON is stored in a new block_models Postgres
table and read alongside the existing (previously write-only)
block_registry table. Fixes a pre-existing face-culling bug as a side
effect of excluding non-cube voxels from the cube mesher's input.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015tKdPZt78zbPUZMXWzKEKt

api/drizzle/0007_block_models.sql

@@ -0,0 +1,8 @@
+CREATE TABLE IF NOT EXISTS "block_models" (
+	"server_id" uuid NOT NULL REFERENCES "servers"("id") ON DELETE CASCADE,
+	"kind" text NOT NULL,
+	"name" text NOT NULL,
+	"json" text NOT NULL,
+	"updated_at" timestamptz NOT NULL DEFAULT now(),
+	PRIMARY KEY ("server_id", "kind", "name")
+);