Add avatar downloading to Bluesky scan user job

- Modified process_user_avatar method to enqueue Domain::UserAvatarJob for avatar downloads
- Made Domain::UserAvatarJob concrete (removed abstract!) with generic HTTP client
- Added smart avatar management: handles new avatars, URL changes, and pending re-enqueues
- Added comprehensive test coverage for all avatar scenarios
- Updated HTTP mocking in specs to use HttpClientMockHelpers pattern
- Fixed caused_by_entry handling for chained HTTP requests
- Updated .cursorrules with proper HTTP mocking documentation including caused_by_entry: :any

The job now automatically downloads user avatars when scanning Bluesky users.

.cursorrules

@@ -10,6 +10,7 @@
 - For instance, if you modify `app/models/domain/post.rb`, run `bin/rspec spec/models/domain/post_spec.rb`. If you modify `app/views/domain/users/index.html.erb`, run `bin/rspec spec/controllers/domain/users_controller_spec.rb`.
 - At the end of a long series of changes, run `just test`.
 - If specs are failing, then fix the failures, and rerun with `bin/rspec <path_to_spec_file>`.
+- If you need to add logging to a Job to debug it, set `quiet: false` on the spec you are debugging.
 
 # Typescript Development
 
@@ -17,6 +18,51 @@
 - Styling is done with Tailwind CSS and FontAwesome.
 - Put new typescript files in `app/javascript/bundles/Main/components/`
 
+# HTTP Mocking in Job Specs
+
+When writing specs for jobs that make HTTP requests, use `HttpClientMockHelpers.init_http_client_mock()` instead of manually creating doubles:
+
+```ruby
+# CORRECT: Use HttpClientMockHelpers
+let(:client_mock_config) do
+  [
+    {
+      uri: "https://example.com/api/first-endpoint",
+      status_code: 200,
+      content_type: "application/json",
+      contents: first_response_body,
+    },
+    {
+      uri: "https://example.com/api/second-endpoint",
+      status_code: 200,
+      content_type: "application/json",
+      contents: second_response_body,
+      caused_by_entry: :any, # Use this for chained requests
+    },
+  ]
+end
+
+before do
+  @log_entries =
+    HttpClientMockHelpers.init_http_client_mock(
+      http_client_mock,
+      client_mock_config,
+    )
+end
+
+# WRONG: Don't create doubles manually
+expect(http_client_mock).to receive(:get).and_return(
+  double(status_code: 200, body: response_body, log_entry: double),
+)
+```
+
+This pattern:
+
+- Creates real HttpLogEntry objects that can be serialized by ActiveJob
+- Follows the established codebase pattern
+- Avoids "Unsupported argument type: RSpec::Mocks::Double" errors
+- Use `caused_by_entry: :any` for HTTP requests that are chained (where one request's log entry becomes the `caused_by_entry` for the next request)
+
 # === BACKLOG.MD GUIDELINES START ===
 
 # Instructions for the usage of Backlog.md CLI Tool