feat(gateway): hivectl gateway user management + fix htpasswdFile assertion

Add `hivectl gateway {create-user,delete-user,list-users}` subcommands for
managing htpasswd files used by gateway Basic auth. Pure Rust bcrypt
(cost 12, $2y$ prefix nginx accepts). No external htpasswd binary required.

Also fix the NixOS module assertion: `cfg.auth ? htpasswdFile` is always
true in the module system (declared options always exist as keys); switch
to `nullOr path; default = null` + `!= null` check so the assertion
actually fires with a useful error when enable=true but no file is set.
Guard bind-mount and nginx config against null to prevent eval errors.

Update docs/gateway.md to show hivectl commands instead of raw htpasswd.

hive-c0re/src/operator_questions.rs

@@ -209,11 +209,7 @@ impl OperatorQuestions {
     ///
     /// Not the target — that's covered by `answer` (responding with
     /// an actual reply, sentinel or otherwise).
-    pub fn cancel(
-        &self,
-        id: i64,
-        canceller: &str,
-    ) -> Result<(String, String, Option<String>)> {
+    pub fn cancel(&self, id: i64, canceller: &str) -> Result<(String, String, Option<String>)> {
         let conn = self.conn.lock().unwrap();
         let row: Option<(String, String, Option<String>, Option<i64>)> = conn
             .query_row(
@@ -232,9 +228,7 @@ impl OperatorQuestions {
             || canceller == hive_sh4re::OPERATOR_RECIPIENT
             || canceller == hive_sh4re::MANAGER_AGENT;
         if !authorised {
-            bail!(
-                "question {id}: '{canceller}' not allowed to cancel (asker = '{asker}')"
-            );
+            bail!("question {id}: '{canceller}' not allowed to cancel (asker = '{asker}')");
         }
         let sentinel = format!("[cancelled by {canceller}]");
         conn.execute(
@@ -289,7 +283,6 @@ impl OperatorQuestions {
         rows.collect::<rusqlite::Result<Vec<_>>>()
             .map_err(Into::into)
     }
-
 }
 
 fn row_to_question(row: &rusqlite::Row<'_>) -> rusqlite::Result<OpQuestion> {