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/body.rs

@@ -23,8 +23,8 @@ pub fn resolve(body: Option<&str>, file: Option<&str>) -> Result<Option<String>>
         if path == "-" {
             return Ok(Some(read_stdin().context("read stdin for --body-file -")?));
         }
-        let s = std::fs::read_to_string(path)
-            .with_context(|| format!("read --body-file {path}"))?;
+        let s =
+            std::fs::read_to_string(path).with_context(|| format!("read --body-file {path}"))?;
         return Ok(Some(s));
     }
     if !std::io::stdin().is_terminal() {