feat: Implement SEO-friendly slug generation and improve geocoding UX

- Add Rails parameterize for server-side slug generation (name-venue-city format)
- Configure client-side slug library with RFC3986 mode for consistency
- Remove slug field from edit forms to prevent URL changes after publication
- Enable image_processing gem for Active Storage variants
- Make geocoding notifications visible indefinitely on promoter event forms
- Add server-side slug generation fallback with uniqueness validation
- Update promoter controller to allow slug only for new events

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/controllers/promoter/events_controller.rb

@@ -134,10 +134,18 @@ class Promoter::EventsController < ApplicationController
   end
 
   def event_params
-    params.require(:event).permit(
-      :name, :slug, :description, :image,
-      :venue_name, :venue_address, :latitude, :longitude,
-      :start_time, :end_time, :featured, :allow_booking_during_event
-    )
+    if action_name == 'create'
+      params.require(:event).permit(
+        :name, :slug, :description, :image,
+        :venue_name, :venue_address, :latitude, :longitude,
+        :start_time, :end_time, :featured, :allow_booking_during_event
+      )
+    else
+      params.require(:event).permit(
+        :name, :description, :image,
+        :venue_name, :venue_address, :latitude, :longitude,
+        :start_time, :end_time, :featured, :allow_booking_during_event
+      )
+    end
   end
 end