feat: add per-user retention settings to users.toml

New [user.retention] TOML sub-block with auto_close_days and
auto_delete_days. Both default to 0, which disables the respective
sweep — a conservative default that preserves current behaviour for
existing deployments and lets admins test auto-close in isolation.

Only parsing + the test User literals are touched here; the actual
lazy sweep consuming these values lands in the next commit.

server/tests/web_test.rs

@@ -21,6 +21,7 @@ fn test_config() -> Arc<Config> {
             web_password: "unused".into(),
             role: "doctor".into(),
             whisper: Default::default(),
+            retention: Default::default(),
         }],
         api_keys: HashMap::from([("test-key".into(), "dr_test".into())]),
         ..Config::test_default()