fix: mount answers/ and default.toml separately, not over /app

Mounting the whole directory to /app overwrites server.py inside the
container, causing "can't open file '/app/server.py'" errors.

Mount the two paths individually instead:
- answer_server_dir/answers → /app/answers
- answer_server_dir/default.toml → /app/default.toml

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/infrastructure/storage.tf

@@ -11,13 +11,14 @@ resource "docker_container" "answer_server" {
 
   network_mode = "host"
 
-  # Mount the answer server root directory which contains:
-  #   default.toml     — fallback answer file
-  #   answers/         — per-MAC answer files
-  # The server expects both at ./default.toml and ./answers/ relative to /app
   volumes {
-    host_path      = var.answer_server_dir
-    container_path = "/app"
+    host_path      = "${var.answer_server_dir}/answers"
+    container_path = "/app/answers"
+  }
+
+  volumes {
+    host_path      = "${var.answer_server_dir}/default.toml"
+    container_path = "/app/default.toml"
   }
 }