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/migrations/2026-03-13-000000_add_key_type/down.sql

@@ -0,0 +1,2 @@
+-- Not reversible without data loss; drop the column to revert
+ALTER TABLE useragent_client DROP COLUMN key_type;

server/crates/arbiter-server/migrations/2026-03-13-000000_add_key_type/up.sql

@@ -0,0 +1 @@
+ALTER TABLE useragent_client ADD COLUMN key_type INTEGER NOT NULL DEFAULT 1;