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/migrations/2026-02-14-171124-0000_init/up.sql

@@ -56,6 +56,7 @@ create table if not exists operator (
 
     share blob not null,
     share_nonce blob not null,
+    share_salt blob not null default (randomblob(32)),
 
     created_at integer not null default(unixepoch ('now')),
     updated_at integer not null default(unixepoch ('now'))

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,
 }

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,
     }