Create anyhow wrapper for unified REST error handling

[whankinsiv]

We currently construct error responses manually in rest_blockfrost for each failure case (parameter parsing, Bech32 conversion, etc.). As suggested by @SupernaviX, introducing a unified AppError type that wraps anyhow::Error and implements From for RESTResponse would simplify error handling across the module. This abstraction will allow REST handlers to use ? error propagation and ensure consistent error responses.