refactor: use AuthStore interface and remove type assertions

Replace runtime type assertions with compile-time safe AuthStore interface.
Add connState struct for cleaner per-connection state management instead
of mutable pointer parameters. Reduce auth challenge TTL from 10min to 2min.

- Add AuthStore interface with CreateAuthChallenge and ValidateAndConsumeChallenge
- Add connState struct for authenticatedPubkey and authChallenge
- Remove fragile type assertion pattern in requireAuth and handleAuth
- Add nil checks for auth store with clear error messages
- Update Handler to have separate auth field
- Wire auth store in main.go when auth is enabled

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