Setup backend

[ianyong]

Ignore all of the github-actions comments; they're from a failed CI job run where the RuboCop configuration file was not properly loaded due to the working directory being wrong.

[github-actions]

Remaining comments which cannot be posted as a review comment to avoid GitHub Rate Limit

rubocop

backend/bin/bundle|112 col 1| Favor modifier if usage when having a single-line body. Another good alternative is the usage of control flow &&/||.
backend/bin/bundle|113 col 21| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/bundle|113 col 32| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/rails|1 col 1| Missing frozen string literal comment.
backend/bin/rails|2 col 23| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/rails|4 col 18| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/rails|5 col 9| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/rake|1 col 1| Missing frozen string literal comment.
backend/bin/rake|2 col 23| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/rake|3 col 18| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/rake|4 col 9| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/setup|1 col 1| Missing frozen string literal comment.
backend/bin/setup|2 col 9| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/spring|1 col 1| Missing frozen string literal comment.
backend/bin/spring|2 col 31| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/spring|2 col 46| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/spring|2 col 67| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/spring|3 col 7| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/spring|4 col 11| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/spring|7 col 58| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/spring|9 col 9| Prefer single-quoted strings when you don't need string interpolation or special symbols.
backend/bin/spring|10 col 13| Prefer single-quoted strings when you don't need string interpolation or special symbols.

[sebastiantoh]

lgtm!