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>

Gemfile

@@ -40,7 +40,7 @@ gem "kamal", require: false
 gem "thruster", require: false
 
 # Use Active Storage variants [https://guides.rubyonrails.org/active_storage_overview.html#transforming-images]
-# gem "image_processing", "~> 1.2"
+gem "image_processing", "~> 1.2"
 
 group :development, :test do
   # See https://guides.rubyonrails.org/debugging_rails_applications.html#debugging-with-the-debug-gem