Feature: Postgres signalling

[TikiTDO]

Summary

Adds an opt-in Rails generator path for Postgres signalling:

• rails generate anycable:setup --postgres-signalling
• configures Rails AnyCable to use broadcast_adapter: postgres
• configures generated anycable.toml with broadcast_adapters = ["http", "postgres"] and pubsub_adapter = "postgres"
• sets the Postgres broadcast/pubsub table names and two wake-up channels expected by anycable-go
• does not generate a Rails migration; anycable-go owns and ensures the signalling schema on startup

The generated config matches the anycable-go server-owned schema and the anycable-rb Postgres broadcast adapter.

Design Notes

• Rails publishes through the anycable-rb Postgres adapter, which calls SQL functions owned by anycable-go.
• anycable-go manages the signalling schema, including anycable_broadcasts, anycable_pubsub, and stream-offset metadata.
• generated config uses the two stable wake-up channels; anycable-go owns their final payload format.
• payload stays text, not jsonb, so AnyCable's exact JSON envelope is preserved and large messages are not constrained by NOTIFY.
• ensure_schema = true keeps setup simple for generated apps while still leaving schema ownership on the server side.

Validation

• bundle exec rspec spec/generators/setup/setup_generator_spec.rb
• bundle exec rubocop lib/generators/anycable/setup/setup_generator.rb spec/generators/setup/setup_generator_spec.rb
• git diff --check

Follow-Up

• The anycable-go PR owns schema creation/validation and consumes this config.
• The anycable-rb PR adds the producer-side postgres broadcast adapter.

Development Note

This PR was prepared with AI assistance and then reviewed, tested, and validated by TikiTDO before submission.

Related PRs

• anycable-go Postgres consumer/pubsub: Feature: Postgres signalling anycable#306
• anycable-rb producer adapter: Feature: Postgres signalling anycable-rb#9
• graphql-anycable support: Add custom subscription store interface graphql-anycable#54

Checklist

• I've added tests for this change
• I've added a Changelog entry
• I've updated documentation