RHEL-Kapitel: Registrierung als Schritt 0, EPEL-Vorbehalt direkt daneben

Beide Hinweise standen bisher nur in Abschnitt 5.1. Wer Kapitel 5 von
oben nach unten abarbeitet, lief aber vorher schon in den dnf-Fehler.
Deshalb jetzt im Hauptablauf selbst:

  - Ein hervorgehobener "Schritt 0" gleich zu Beginn des Kapitels mit
    den Pruefbefehlen (subscription-manager status, dnf repolist) und
    der Registrierung ueber die kostenlose Developer-Subskription.
    Dazu der Hinweis, dass RHEL 9 Simple Content Access nutzt und
    "subscription-manager attach" entfaellt, sowie die Feststellung,
    dass Rocky/Alma/CentOS Stream/Fedora nichts davon brauchen.

  - Direkt beim epel-release-Befehl der Vorbehalt, dass EPEL eine
    funktionierende Subskription voraussetzt: das Paket konfiguriert
    nur die Quelle, Abhaengigkeiten kommen weiter aus BaseOS/AppStream.
    EPEL ist damit kein Ersatz fuer die Registrierung. Bleibt
    podman-compose deswegen unerreichbar, fuehrt der Quadlet-Weg aus
    Kapitel 6 ohne Compose zum Ziel.

Die Aufzaehlung dessen, was install.sh tut, entspricht jetzt dem
tatsaechlichen Ablauf: Repository-Pruefung zuerst, dialog mit Rueckfall
auf newt.

Beide Sprachfassungen gepflegt.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

README_en.md

@@ -187,6 +187,31 @@ worthwhile because it additionally starts **all** sites
 Docker is not available on Red Hat systems – **Podman** is used instead. The
 compose file is the same one.
 
+> ### Step 0 on RHEL: check the subscription
+>
+> **An unregistered RHEL installs no packages at all** – neither `podman` nor
+> `git`, `dialog` or `tar`. `dnf` merely reports *"This system is not registered
+> with an entitlement server"*. This is the most common reason why `install.sh`
+> does not get through on a fresh RHEL.
+>
+> ```bash
+> # is the system registered?
+> sudo subscription-manager status      # -> "Overall Status: Current" = fine
+> sudo dnf repolist                     # -> must not be empty
+>
+> # if not: create a free account at https://developers.redhat.com
+> # (Developer Subscription for Individuals, up to 16 systems) and then:
+> sudo subscription-manager register --username <user>
+> ```
+>
+> RHEL 9 uses *Simple Content Access*, so `subscription-manager attach` is **no
+> longer** required. Without an account it also works from the installation DVD
+> or by switching to Rocky/Alma – all three routes are covered in detail in
+> [section 5.1](#5-installation-on-rhelrockyalmafedora-podman).
+>
+> **Rocky Linux, AlmaLinux, CentOS Stream and Fedora** need none of this – just
+> read on.
+
 ```bash
 sudo mkdir -p /opt/checkmk-stack
 sudo cp -r ./* ./.env.example /opt/checkmk-stack/
@@ -197,20 +222,36 @@ sudo ./install.sh --with-systemd
 
 `install.sh` installs:
 
-1. `podman`, `tar`, `dialog`, `openssl`
-2. `podman-compose` – attempted in this order:
+1. first checks whether a usable repository exists at all, and otherwise names
+   the three routes from [section 5.1](#5-installation-on-rhelrockyalmafedora-podman)
+2. `podman`, `tar`, `openssl`
+3. `dialog` for the ncurses interface – if that fails, `newt` (`whiptail`) is
+   used instead; the scripts support either
+4. `podman-compose` – attempted in this order:
    distribution repository → EPEL → `pip3 install podman-compose`
-3. sets up the systemd unit (**always** with Podman, see the next chapter)
+5. sets up the systemd unit (**always** with Podman, see the next chapter)
 
 Done manually, that is:
 
 ```bash
-sudo dnf install -y podman podman-compose tar dialog openssl
-# if podman-compose is not in the repository:
+sudo dnf install -y podman podman-compose tar openssl
+sudo dnf install -y dialog || sudo dnf install -y newt   # dialog: AppStream, newt: BaseOS
+
+# if podman-compose is not in the repository - add EPEL:
 sudo dnf install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-9.noarch.rpm
 sudo dnf install -y podman-compose
 ```
 
+> **EPEL requires a working subscription.** The `epel-release` package only
+> configures the EPEL source; its dependencies and everything installed from it
+> afterwards still come from Red Hat's BaseOS/AppStream. On an unregistered
+> system even the command above therefore fails – EPEL is **not** a substitute
+> for registering.
+>
+> If `podman-compose` stays out of reach because of this, it is no obstacle:
+> with **Quadlet** ([chapter 6, option B](#6-starting-automatically-after-a-host-reboot-podman))
+> the stack runs entirely without compose, straight through systemd.
+
 Starting up:
 
 ```bash