feat(gateway): htpasswd Basic auth — close #1010

Replaces the earlier PAM+binary approach with nginx's built-in
`auth_basic` module. No new binary, no new systemd service, no PAM.

New option `services.hyperhive.gateway.auth`:
- `enable` — off by default
- `htpasswdFile` — host path to an htpasswd file (required when enable)
- `realm` — WWW-Authenticate realm string (default "hyperhive");
  restricted to `strMatching "[^\"$]*"` to prevent nginx config injection

When enabled:
- the parent directory of `htpasswdFile` is bind-mounted read-only
  into the gateway container at `/run/gateway-auth/`
- the `"/"` proxy location gets `auth_basic` + `auth_basic_user_file`

Create credentials: `htpasswd -Bc /path/to/file alice` (BCrypt).
See `docs/gateway.md` ("HTTP Basic auth") for the full setup guide.

Cargo.toml

@@ -16,7 +16,6 @@ must_use_candidate = "allow"
 
 [workspace.dependencies]
 anyhow = "1"
-pam = "0.8"
 axum = { version = "0.8", features = ["ws"] }
 base64 = "0.22"
 clap = { version = "4", features = ["derive"] }