Reduce repo cache size

As assets are self-contained in their own table, and shielded with heavy
CDN + in-memory caching, there's no point in keeping a very large cache.

config/runtime.exs

@@ -100,8 +100,8 @@ if config_env() == :prod do
 
   config :music_library, MusicLibrary.Repo,
     database: database_path,
-    # 128MB * pool_size = base memory usage
-    cache_size: -128_000,
+    # 64MB * pool_size = base memory usage
+    cache_size: -64_000,
     pool_size: String.to_integer(System.get_env("POOL_SIZE") || "5"),
     show_sensitive_data_on_connection_error: false