feat(status): Display external-url in juju status when configured

Enhance the status message to show the configured external-url instead
of the auto-detected IP:PORT when external-url is set.

Changes:
- Modified _get_web_url() to prioritize external-url configuration
- Falls back to auto-detected http://IP:PORT when external-url is empty
- Maintains backward compatibility with existing deployments

Behavior:
- When external-url is set: Shows 'Web server ready (v7.14.2) <external-url>'
- When external-url is empty: Shows 'Web server ready (v7.14.2) http://IP:PORT'

This ensures the status message reflects the actual external URL that
Concourse uses for webhooks and OAuth redirects, improving visibility
and reducing confusion when running behind proxies or with custom domains.

Author: fourdollars

src/charm.py

@@ -1792,9 +1792,17 @@ def _get_installed_concourse_version(self) -> Optional[str]:
         return None
 
     def _get_web_url(self) -> Optional[str]:
-        """Get the web server URL (http://IP:PORT) for status display"""
+        """Get the web server URL for status display
+
+        Prioritizes external-url config, falls back to auto-detected IP:PORT
+        """
         try:
-            # Get the unit's IP address from network binding
+            # Check if external-url is configured
+            external_url = self.config.get("external-url", "").strip()
+            if external_url:
+                return external_url
+
+            # Auto-detect: Get the unit's IP address from network binding
             # Try multiple bindings in order of preference
             binding = None
             for binding_name in ["tsa", "peers", ""]: