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/models.rs

@@ -285,6 +285,7 @@ pub struct Operator {
     pub id: OperatorId,
     pub share: Vec<u8>,
     pub share_nonce: Vec<u8>,
+    pub share_salt: Vec<u8>,
     pub created_at: SqliteTimestamp,
     pub updated_at: SqliteTimestamp,
 }