3.11.18

release: v3.11.18 — Tina4.background helper + dev-admin bundle refresh

Closes the audit-known parity gap: Ruby was missing the periodic
callback helper that Python (background()) and PHP ($app->background())
already shipped. Users had to fall back to ServiceRunner cron-style
registration for what should be a one-liner periodic task.

• New Tina4::Background module + Tina4.background(interval:, &block)
  module-level DSL. Spawns a tracked thread per task, rescues errors
  inside the loop so one bad tick doesn't kill the rest.
• Wired into Tina4::Shutdown so SIGTERM/SIGINT joins or kills tasks
  cleanly.
• 7 new RSpec examples in spec/background_spec.rb covering periodic
  invocation, error isolation, stop semantics, graceful shutdown.

Bundle refresh ships per-framework chat-prompt overlay (Tina4::Router
examples instead of Python decorators), "generate image of …" intent
shortcut, optional RAG retrieval, and chat history persisted across
page reloads.