feat(multiplayer): persist ranked arena allocations

server/migrations/0008_match_arena_paths.sql

@@ -0,0 +1,8 @@
+-- Persist the matcher-selected arena through acceptance and allocation. NULL
+-- remains valid for legacy/casual rows while ranked proposals always write a
+-- server-owned floor-goal path.
+ALTER TABLE proposals
+    ADD COLUMN match_arena_path TEXT;
+
+ALTER TABLE matches
+    ADD COLUMN arena_path TEXT;