Fix Rails database setup commands and add prerequisite steps

[coderNini]

Summary

The rails db:setup command fails for at least some developers. The developer documentation should provide working alternatives and make prerequisites explicit.

Proposed Changes

• Add bundle exec rails db:setup as the recommended (or fallback) command in place of rails db:setup
• Add a step to run bundle install before bundle exec rails db:setup
• Add a note that PostgreSQL must be running before executing any db setup commands, and explain how to verify/start it

Make changes in #408 (and refer to comments for further detail)