Refactor email delivery and background jobs system

- Switch from SolidQueue to async job processor for simpler background job handling
- Remove SolidQueue gem and related configuration files
- Add letter_opener gem for development email preview
- Fix invitation email template issues (invitation_login_token method and route helper)
- Configure SMTP settings via environment variables in application.rb
- Add email delivery configuration banner on admin users page
- Improve admin users page with inline action buttons and SMTP configuration warnings
- Update development and production environments to use async processor
- Add helper methods to detect SMTP configuration and filter out localhost settings

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

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

Gemfile.lock

@@ -100,6 +100,8 @@ GEM
       rack-test (>= 0.6.3)
       regexp_parser (>= 1.5, < 3.0)
       xpath (~> 3.2)
+    childprocess (5.1.0)
+      logger (~> 1.5)
     chunky_png (1.4.0)
     concurrent-ruby (1.3.5)
     connection_pool (2.5.4)
@@ -113,8 +115,6 @@ GEM
     ed25519 (1.4.0)
     erb (5.1.1)
     erubi (1.13.1)
-    et-orbi (1.4.0)
-      tzinfo
     ffi (1.17.2-aarch64-linux-gnu)
     ffi (1.17.2-aarch64-linux-musl)
     ffi (1.17.2-arm-linux-gnu)
@@ -122,9 +122,6 @@ GEM
     ffi (1.17.2-arm64-darwin)
     ffi (1.17.2-x86_64-linux-gnu)
     ffi (1.17.2-x86_64-linux-musl)
-    fugit (1.12.1)
-      et-orbi (~> 1.4)
-      raabro (~> 1.4)
     globalid (1.3.0)
       activesupport (>= 6.1)
     i18n (1.14.7)
@@ -159,6 +156,12 @@ GEM
       thor (~> 1.3)
       zeitwerk (>= 2.6.18, < 3.0)
     language_server-protocol (3.17.0.5)
+    launchy (3.1.1)
+      addressable (~> 2.8)
+      childprocess (~> 5.0)
+      logger (~> 1.6)
+    letter_opener (1.10.0)
+      launchy (>= 2.2, < 4)
     lint_roller (1.1.0)
     logger (1.7.0)
     loofah (2.24.1)
@@ -225,7 +228,6 @@ GEM
     public_suffix (6.0.2)
     puma (7.1.0)
       nio4r (~> 2.0)
-    raabro (1.4.0)
     racc (1.8.1)
     rack (3.2.3)
     rack-session (2.1.1)
@@ -329,13 +331,6 @@ GEM
       activejob (>= 7.2)
       activerecord (>= 7.2)
       railties (>= 7.2)
-    solid_queue (1.2.2)
-      activejob (>= 7.1)
-      activerecord (>= 7.1)
-      concurrent-ruby (>= 1.3.1)
-      fugit (~> 1.11)
-      railties (>= 7.1)
-      thor (>= 1.3.1)
     sqlite3 (2.7.4-aarch64-linux-gnu)
     sqlite3 (2.7.4-aarch64-linux-musl)
     sqlite3 (2.7.4-arm-linux-gnu)
@@ -416,6 +411,7 @@ DEPENDENCIES
   jbuilder
   jwt (~> 3.1)
   kamal
+  letter_opener
   propshaft
   puma (>= 5.0)
   rails (~> 8.1.0)
@@ -425,7 +421,6 @@ DEPENDENCIES
   selenium-webdriver
   solid_cable
   solid_cache
-  solid_queue
   sqlite3 (>= 2.1)
   stimulus-rails
   tailwindcss-rails