doc: add a visualization of the database models

Re-generate the schema using `bin/rake erd`.

NB: there's a way to update the PDF automatically after each migration.
But it requires `graphviz` to be installed locally, which I'm not
sure I want to require by default.

.erdconfig

@@ -0,0 +1,4 @@
+indirect: false
+disconnected: false
+filename: doc/database_models
+exclude: "ActionText::RichText,ActiveStorage::Blob,ActiveStorage::Attachment"

Gemfile

@@ -100,6 +100,7 @@ group :development do
   gem 'brakeman', require: false
   gem 'haml-lint'
   gem 'letter_opener_web'
+  gem 'rails-erd', require: false # generates `doc/database_models.pdf`
   gem 'rubocop', require: false
   gem 'rubocop-rails_config'
   gem 'rubocop-rspec-focused', require: false

Gemfile.lock

@@ -156,6 +156,7 @@ GEM
       rubyzip (>= 1.3.0, < 3)
     chartkick (3.4.0)
     childprocess (3.0.0)
+    choice (0.2.0)
     chunky_png (1.3.11)
     clamav-client (3.2.0)
     coderay (1.1.2)
@@ -522,6 +523,11 @@ GEM
     rails-dom-testing (2.0.3)
       activesupport (>= 4.2.0)
       nokogiri (>= 1.6)
+    rails-erd (1.6.0)
+      activerecord (>= 4.2)
+      activesupport (>= 4.2)
+      choice (~> 0.2.0)
+      ruby-graphviz (~> 1.2)
     rails-html-sanitizer (1.3.0)
       loofah (~> 2.3)
     rails-i18n (6.0.0)
@@ -611,6 +617,8 @@ GEM
       rubocop (>= 0.51)
     ruby-debug-ide (0.7.2)
       rake (>= 0.8.1)
+    ruby-graphviz (1.2.5)
+      rexml
     ruby-progressbar (1.10.1)
     ruby2_keywords (0.0.2)
     ruby_parser (3.14.2)
@@ -826,6 +834,7 @@ DEPENDENCIES
   rack-mini-profiler
   rails (= 6.0.3.2)
   rails-controller-testing
+  rails-erd
   rails-i18n
   rake-progressbar
   react-rails

doc/README.md

@@ -0,0 +1,8 @@
+## database_models.pdf
+
+The database models document is generated using the `rails-erd` gem.
+
+To update the generated PDF file:
+
+1. Install `graphviz` (e.g. `brew install graphviz`)
+2. Run `bin/rake erd`