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/models/event.rb

@@ -26,6 +26,7 @@ class Event < ApplicationRecord
 
   
   # === Callbacks ===
+  before_validation :generate_slug, if: :should_generate_slug?
   before_validation :geocode_address, if: :should_geocode_address?
   before_update :handle_image_replacement, if: :image_attached?
 
@@ -62,6 +63,71 @@ class Event < ApplicationRecord
 
   # === Instance Methods ===
 
+  # Generate SEO-friendly slug from name, venue name, and city
+  def generate_slug
+    return if name.blank? && venue_name.blank? && venue_address.blank?
+
+    # Extract city from venue address
+    city = extract_city_from_address(venue_address)
+
+    # Build slug parts
+    slug_parts = []
+    slug_parts << name if name.present?
+    slug_parts << venue_name if venue_name.present?
+    slug_parts << city if city.present?
+
+    # Generate slug using Rails' parameterize
+    slug_value = slug_parts.join('-').parameterize
+
+    # Ensure minimum length
+    if slug_value.length < 3
+      slug_value = "event-#{Time.current.to_i}".parameterize
+    end
+
+    # Make sure slug is unique
+    base_slug = slug_value
+    counter = 1
+    while Event.where.not(id: id).where(slug: slug_value).exists?
+      slug_value = "#{base_slug}-#{counter}".parameterize
+      counter += 1
+    end
+
+    self.slug = slug_value
+  end
+
+  # Check if slug should be generated
+  def should_generate_slug?
+    # Generate slug if it's blank or if it's a new record
+    slug.blank? || new_record?
+  end
+
+  # Extract city from address
+  def extract_city_from_address(address)
+    return "" if address.blank?
+
+    # Look for French postal code pattern (5 digits) + city
+    match = address.match(/(\d{5})\s+([^,]+)/)
+    if match
+      return match[2].strip
+    end
+
+    # Fallback: extract last part after comma (assume it's city)
+    parts = address.split(',')
+    if parts.length > 1
+      return parts[parts.length - 1].strip
+    end
+
+    # Another fallback: look for common French city indicators
+    city_indicators = ["Paris", "Lyon", "Marseille", "Toulouse", "Nice", "Nantes", "Strasbourg", "Montpellier", "Bordeaux", "Lille"]
+    for city in city_indicators
+      if address.downcase.include?(city.downcase)
+        return city
+      end
+    end
+
+    ""
+  end
+
   # Get image URL prioritizing old image field if it exists
   def display_image_url
     # First check if old image field exists and has a value