The existing *.env pattern did not match deploy/.env.prod, which holds
database, Zitadel and OIDC credentials. Also ignore deploy/secrets/,
mounted into the api and worker containers.
- Move all .NET (6 projects, solution, Directory.*.props, nuget.config)
into backend/ so the monorepo root can host non-.NET projects later
- Remove throwaway test artifacts: player.html, pl.json
- Trim dead Flutter rules from .gitignore
Build from backend/Musify.slnx: 0 warnings, 0 errors.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>