`ban_message` default uses a non-standard `banned:` prefix

[nogringo]

banned: is not one of the NIP-01 OK prefixes (duplicate, pow, blocked, rate-limited, invalid, restricted, error, auth-required), so a client that keys its retry behavior off the prefix can't classify the rejection.

• src/filters/ban.rs:69 : "banned: too many violations, try again later"
• noteguard.toml:13 : ban_message = "banned: too many rate-limit violations, try again later"

[linear]

SER-100