Default in-memory token store backed by ETS, supervised by a GenServer.

Tokens are keyed by {store_id, token_type}, where each TruelayerClient instance receives a unique store_id (make_ref/0), providing complete isolation between multiple clients in the same node.

Characteristics

• O(1) reads via ETS with read_concurrency: true
• Atomic insert_new for race-free concurrent writes
• Tokens survive process crashes (ETS table is owned by the GenServer, not the calling process)
• Data is lost on node restart — use a Redis-backed store for persistence

Started automatically by TruelayerClient.Application.