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-forge/src/verbs/comments.rs

@@ -105,10 +105,7 @@ fn fetch_tail(client: &Client, repo: &str, number: u64, n: usize) -> Result<Vec<
         return Ok(Vec::new());
     }
     let issue = client.get_json(&format!("/repos/{repo}/issues/{number}"))?;
-    let total = issue
-        .get("comments")
-        .and_then(Value::as_u64)
-        .unwrap_or(0) as usize;
+    let total = issue.get("comments").and_then(Value::as_u64).unwrap_or(0) as usize;
     if total == 0 {
         return Ok(Vec::new());
     }