ExGram.BotInit hook that initialises the FSM storage backend at bot startup.
This hook is automatically registered by use ExGram.FSM — you do not need
to add it manually. It calls storage.init/2 once per bot during the startup
sequence, before the bot begins processing updates.
The hook receives the bot name atom via opts[:bot] (provided by the ExGram
dispatcher) and the storage module via opts[:storage] (injected by
use ExGram.FSM).