Feature: Postgres signalling

[TikiTDO]

Summary

Adds a Postgres broadcast adapter to AnyCable Ruby:

• broadcast_adapter: postgres
• calls the SQL functions owned by anycable-go: anycable_publish(stream, payload, meta) and anycable_remote_command(payload, meta)
• preserves the full AnyCable JSON envelope as opaque text for the server-owned schema
• supports normal stream broadcasts, remote commands, and batched broadcasts by decomposing the batch into one SQL function call per message
• documents the adapter/config and adds RBS signatures, focused specs, and a changelog entry

The adapter pairs with anycable-go's Postgres implementation. PostgreSQL LISTEN/NOTIFY stays in anycable-go as a wake-up path; the Ruby producer does not manage signalling tables, triggers, or schema validation.

Config

New config key:

• postgres_url (ANYCABLE_POSTGRES_URL, defaults through DATABASE_URL before falling back to localhost)

Validation

• bundle exec rspec spec/anycable/broadcast_adapters/postgres_spec.rb spec/anycable/config_spec.rb
• bundle exec rubocop lib/anycable/broadcast_adapters/postgres.rb lib/anycable/config.rb spec/anycable/broadcast_adapters/postgres_spec.rb spec/anycable/config_spec.rb
• git diff --check

Follow-Up

• The anycable-go PR owns schema creation/validation and consumes the SQL-function writes.
• The anycable-rails PR configures generated apps for this server-owned schema.

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-rails setup generator: Feature: Postgres signalling anycable-rails#223
• 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