Switch sign-in flow from GitHub to codebar auth

[mroderick]

What

Now that /auth/codebar has been proven in production, this change routes all user-facing sign-in entry points to the codebar auth provider instead of GitHub.

Changes

• ApplicationController#redirect_path now returns /auth/codebar.
• Navigation "Sign in" link and the Terms & Conditions login link use the helper instead of a hardcoded GitHub path.
• Remaining hardcoded redirects in AuthServicesController#new and TermsAndConditionsController#update point to /auth/codebar.
• Updated user-facing copy in config/locales/en.yml from "GitHub" to "codebar".
• Updated specs, fabricators, and OmniAuth test mocks to exercise the codebar provider.

Verification

• make test passes: 1153 examples, 0 failures.
• RuboCop and HAML lint offences are pre-existing only.

Notes

• AuthServicesController#destroy is intentionally unchanged; it overrides redirect_path to return :services.
• No new routes, constants, or helpers were introduced.