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.lock

@@ -122,6 +122,13 @@ GEM
     erubi (1.13.1)
     et-orbi (1.3.0)
       tzinfo
+    ffi (1.17.2-aarch64-linux-gnu)
+    ffi (1.17.2-aarch64-linux-musl)
+    ffi (1.17.2-arm-linux-gnu)
+    ffi (1.17.2-arm-linux-musl)
+    ffi (1.17.2-x86_64-darwin)
+    ffi (1.17.2-x86_64-linux-gnu)
+    ffi (1.17.2-x86_64-linux-musl)
     fugit (1.11.2)
       et-orbi (~> 1, >= 1.2.11)
       raabro (~> 1.4)
@@ -129,6 +136,9 @@ GEM
       activesupport (>= 6.1)
     i18n (1.14.7)
       concurrent-ruby (~> 1.0)
+    image_processing (1.14.0)
+      mini_magick (>= 4.9.5, < 6)
+      ruby-vips (>= 2.0.17, < 3)
     io-console (0.8.1)
     irb (1.15.2)
       pp (>= 0.6.0)
@@ -177,6 +187,8 @@ GEM
       net-smtp
     marcel (1.0.4)
     matrix (0.4.3)
+    mini_magick (5.3.1)
+      logger
     mini_mime (1.1.5)
     minitest (5.25.5)
     minitest-reporters (1.7.1)
@@ -333,6 +345,9 @@ GEM
       rubocop-performance (>= 1.24)
       rubocop-rails (>= 2.30)
     ruby-progressbar (1.13.0)
+    ruby-vips (2.2.5)
+      ffi (~> 1.12)
+      logger
     ruby2_keywords (0.0.5)
     rubyzip (3.0.2)
     securerandom (0.4.1)
@@ -429,6 +444,7 @@ DEPENDENCIES
   debug
   devise (~> 4.9)
   dotenv-rails
+  image_processing (~> 1.2)
   jbuilder
   jsbundling-rails
   kamal