feat: add lightweight /health endpoint (#13386)

The existing /api health check endpoint creates a new Redis connection
on every request and checks both Redis and Postgres availability. During
peak traffic, this creates unnecessary load and can cause cascading
failures when either service is slow - instances get marked unhealthy,
traffic shifts to remaining instances, which then also fail health
checks.

The new /health endpoint:
  - Returns immediately with 200 {"status":"woot"}
  - Skips all middleware and authentication
  - No Redis or Postgres dependency
- Suitable for health checks that only need to verify the web server is
responding

app/controllers/health_controller.rb

@@ -0,0 +1,7 @@
+# Inherits from ActionController::Base to skip all middleware,
+# authentication, and callbacks. Used for health checks
+class HealthController < ActionController::Base # rubocop:disable Rails/ApplicationController
+  def show
+    render json: { status: 'woot' }
+  end
+end