feat(user-agent-auth): add RSA and ECDSA auth key types

Extend user-agent authentication to support Ed25519, ECDSA (secp256k1), and RSA (PSS+SHA-256) with minimal protocol and storage changes. Add key_type to auth requests and useragent_client, update key parsing/signature verification paths, and keep backward compatibility by treating UNSPECIFIED as Ed25519.

server/crates/arbiter-server/src/db/schema.rs

@@ -153,6 +153,7 @@ diesel::table! {
         public_key -> Binary,
         created_at -> Integer,
         updated_at -> Integer,
+        key_type -> Integer,
     }
 }