fix(source/*): update keyring installation method

repo.multiflexi.eu no longer serves multiflexi-archive-keyring.deb (404);
switch install.rst, quickstart.rst, and troubleshooting.rst to fetching
KEY.gpg directly, matching the fix already applied in the
validate-multiflexi-app GitHub Action.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Author: Vitexus

source/install.rst

@@ -49,11 +49,10 @@ First, add the MultiFlexi repository to your system's package sources.
     sudo apt install -y lsb-release apt-transport-https bzip2 ca-certificates curl
 
     # Add GPG key
-    curl -sSLo /tmp/multiflexi-archive-keyring.deb https://repo.multiflexi.eu/multiflexi-archive-keyring.deb
-    sudo dpkg -i /tmp/multiflexi-archive-keyring.deb
+    sudo curl -fsSL https://repo.multiflexi.eu/KEY.gpg -o /usr/share/keyrings/multiflexi-archive-keyring.gpg
 
     # Add repository source
-    echo "deb [signed-by=/usr/share/keyrings/repo.multiflexi.eu.gpg] https://repo.multiflexi.eu/ $(lsb_release -sc) main" | sudo tee /etc/apt/sources.list.d/multiflexi.list
+    echo "deb [signed-by=/usr/share/keyrings/multiflexi-archive-keyring.gpg] https://repo.multiflexi.eu/ $(lsb_release -sc) main" | sudo tee /etc/apt/sources.list.d/multiflexi.list
 
     # Refresh package lists
     sudo apt update

source/quickstart.rst

@@ -22,9 +22,8 @@ Installation (5 minutes)
 
 .. code-block:: bash
 
-   curl -sSLo /tmp/multiflexi-archive-keyring.deb https://repo.multiflexi.eu/multiflexi-archive-keyring.deb
-   sudo dpkg -i /tmp/multiflexi-archive-keyring.deb
-   echo "deb [signed-by=/usr/share/keyrings/repo.multiflexi.eu.gpg] https://repo.multiflexi.eu/ $(lsb_release -sc) main" | sudo tee /etc/apt/sources.list.d/multiflexi.list
+   sudo curl -fsSL https://repo.multiflexi.eu/KEY.gpg -o /usr/share/keyrings/multiflexi-archive-keyring.gpg
+   echo "deb [signed-by=/usr/share/keyrings/multiflexi-archive-keyring.gpg] https://repo.multiflexi.eu/ $(lsb_release -sc) main" | sudo tee /etc/apt/sources.list.d/multiflexi.list
    sudo apt update
 
 **2. Install MultiFlexi with MySQL**

source/troubleshooting.rst

@@ -169,10 +169,9 @@ APT Repository Not Found
 .. code-block:: bash
 
    # Re-add the repository
-   curl -sSLo /tmp/multiflexi-archive-keyring.deb \
-     https://repo.multiflexi.eu/multiflexi-archive-keyring.deb
-   sudo dpkg -i /tmp/multiflexi-archive-keyring.deb
-   echo "deb [signed-by=/usr/share/keyrings/repo.multiflexi.eu.gpg] \
+   sudo curl -fsSL https://repo.multiflexi.eu/KEY.gpg \
+     -o /usr/share/keyrings/multiflexi-archive-keyring.gpg
+   echo "deb [signed-by=/usr/share/keyrings/multiflexi-archive-keyring.gpg] \
      https://repo.multiflexi.eu/ $(lsb_release -sc) main" | \
      sudo tee /etc/apt/sources.list.d/multiflexi.list
    sudo apt update