Link new plugin notification to specific plugin page

[simonhamp]

Summary

• Email notification "View Plugin" action now links to the specific plugin page (/plugins/{vendor}/{package}) instead of the generic /plugins directory
• Database notification data now includes action_url and action_label fields
• Notifications Blade template renders a clickable action button when action_url is present (generic pattern, works for any notification)

Test plan

• New test: test_mail_action_links_to_plugin_page asserts correct URL in email
• Extended test_database_notification_contains_plugin_data to assert action_url and action_label
• All 8 notification tests pass

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