feat(server::db): add share_salt column to operator table

Each operator row now stores a 32-byte random salt used to derive the
per-operator share encryption key from their passphrase (Argon2 KDF).

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

@@ -157,6 +157,7 @@ diesel::table! {
         id -> Nullable<Integer>,
         share -> Binary,
         share_nonce -> Binary,
+        share_salt -> Binary,
         created_at -> Integer,
         updated_at -> Integer,
     }