UX: auto-install plugins when enabling from UI

enable_plugin() now auto-installs available plugins before enabling them.
This fixes the UX issue where clicking Enable on an AVAILABLE plugin would
fail with 'Plugin is not installed'. Now it installs automatically (runs
migrations, seeds permissions) then enables it.

Before: User had to manually install via CLI first
After: Enable button works on AVAILABLE plugins (one-click install+enable)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

shopdb/plugins/__init__.py

@@ -468,10 +468,13 @@ class PluginManager:
         return True
 
     def enable_plugin(self, name: str) -> bool:
-        """Enable a disabled plugin."""
+        """Enable a disabled plugin. Auto-installs if available but not installed."""
+        # Auto-install if available but not installed (UX: enabling should just work)
         if not self.registry.is_installed(name):
-            logger.error(f"Plugin {name} is not installed")
-            return False
+            logger.info(f"Plugin {name} not installed, installing first")
+            if not self.install_plugin(name):
+                logger.error(f"Failed to install plugin {name}")
+                return False
 
         if self.registry.is_enabled(name):
             logger.info(f"Plugin {name} is already enabled")