Use a single database for primary, cache, queue, cable

[raghubetina]

Summary

Rails 8 ships config/database.yml with four separately-configured roles in production (primary, cache, queue, cable) so Solid Cache, Solid Queue, and Solid Cable can each live in their own database. The default already points all four roles at the same DATABASE_URL with separate migration paths — sharing one Render Postgres saves money but keeps the conceptual overhead of four config blocks, three empty stub schema files, and three nonexistent migration directories.

The migrations actually live in db/migrate/ and db/schema.rb already contains all the SC/SQ/SCable tables, so the multi-role config was purely cosmetic. This PR collapses it to a single role.

Changes:

• config/database.yml: replace the four-role production block with a single <<: *default + url: pair.
• config/cache.yml: drop database: cache so SolidCache::Record inherits ApplicationRecord's connection.
• config/cable.yml: drop the connects_to block so Solid Cable inherits the primary connection.
• Delete db/cable_schema.rb, db/cache_schema.rb, db/queue_schema.rb — empty (version: 0) stubs only meaningful when each role has its own database.

The same single-DB pattern is already in production on aplace.app and has been stable since the async-mode rollout.

Test plan

• RAILS_ENV=production bin/rails runner confirms ApplicationRecord, SolidQueue::Record, SolidCache::Record, and SolidCable::Record all report connection_db_config.name == "primary".
• Cross-checked the Rails multi-DB guide for cleanup items: no connects_to, database_selector, database_resolver, shard_selector, or per-DB schema cache files anywhere in config/ or app/.
• No remaining references to cache_migrate, queue_migrate, or cable_migrate directories (which never existed on disk).
• On first deploy after merge, confirm a fresh Render Postgres has all SC/SQ/SCable tables created in the primary database from db/schema.rb.