Include example backend implementation for the connect feature

config/examples/connect_backend_ruby/Gemfile

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+source "https://rubygems.org"
+
+gem "sinatra"
+gem "json"
+gem "puma"

config/examples/connect_backend_ruby/Gemfile.lock

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+GEM
+  remote: https://rubygems.org/
+  specs:
+    json (2.6.2)
+    mustermann (3.0.0)
+      ruby2_keywords (~> 0.0.1)
+    nio4r (2.5.8)
+    puma (6.0.0)
+      nio4r (~> 2.0)
+    rack (2.2.4)
+    rack-protection (3.0.4)
+      rack
+    ruby2_keywords (0.0.5)
+    sinatra (3.0.4)
+      mustermann (~> 3.0)
+      rack (~> 2.2, >= 2.2.4)
+      rack-protection (= 3.0.4)
+      tilt (~> 2.0)
+    tilt (2.0.11)
+
+PLATFORMS
+  arm64-darwin-21
+
+DEPENDENCIES
+  json
+  puma
+  sinatra
+
+BUNDLED WITH
+   2.3.9

config/examples/connect_backend_ruby/README.md

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+# connect-backend-ruby
+
+Example Ruby backend for Pgweb Connect feature
+
+## Usage
+
+Install and run the backend:
+
+```bash
+bundle install
+ruby main.rb
+```
+
+Configure pgweb:
+
+```bash
+pgweb --sessions --connect-backend=http://localhost:4567 --connect-token=test
+```

config/examples/connect_backend_ruby/config.ru

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+require "bundler/setup"
+require "./main"
+
+run Sinatra::Application

config/examples/connect_backend_ruby/main.rb

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+require "sinatra"
+
+# Authentication token
+$token = "test"
+
+# List of all availble resources
+$resources = {
+  "id1" => "postgres://localhost:5432/db1?sslmode=disable",
+  "id2" => "postgres://localhost:5432/db2?sslmode=disable",
+  "id3" => "postgres://localhost:5432/db3?sslmode=disable"
+}
+
+helpers do
+  def error(code, message)
+    halt(code, JSON.dump(error: message))
+  end
+end
+
+before do
+  content_type :json
+end
+
+post "/" do
+  req = JSON.load(request.body) || {}
+
+  unless req["resource"]
+    halt 404, "Resource ID required"
+  end
+
+  # Check the resource
+  resource = $resources[req["resource"]]
+  if !resource
+    halt 404, "Invalid resource ID"
+  end
+
+  # Return connection credentials
+  JSON.dump(
+    database_url: resource
+  )
+end