Document SOLID_QUEUE_IN_PUMA=true in .env.sample for dev

Without it, deliver_later jobs (e.g. invitation emails) sit in the
queue unprocessed since no worker runs in development. The Puma
plugin (config/puma.rb) already activates when this env var is set.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: dadachi

.env.sample

@@ -13,3 +13,8 @@
 
 HOST=192.168.1.21
 PORT=3000
+
+# Run Solid Queue inside Puma so background jobs (e.g. invitation emails
+# via deliver_later) are processed in development. Mirrors production
+# (see render.yaml).
+SOLID_QUEUE_IN_PUMA=true