StateManager() at module level raises BackendNotFoundError with a message that does not say what to do

[allen0099]

Problem

Creating StateManager() at module level, the natural place for it, fails when the backend is set in the app's lifespan (the documented place):

BackendNotFoundError: No instance set for proxy BackendProxy

The message does not say what to do (call BackendProxy.set(...) first, or create the manager after the backend is set).

Proposal

• Resolve the backend lazily on first use rather than in __init__, so module-level construction works.
• Make the BackendNotFoundError message actionable: name BackendProxy.set(...) and point to the setup docs.