Clean up backend and web-player leftovers

Remove dead scaffold/config cruft and stale references left over from
recent refactors: dotnet's WebApi.http template file, the AppSettings
PascalCase/Dockerfile workaround (renamed to appsettings.json to match
convention), empty directories left behind by the auth-route and
folder-structure moves, a stale ESLint ignore, and the now-fully-applied
frontend refactor plan doc.

backend/Musify.Api.Dockerfile

@@ -12,10 +12,6 @@ RUN dotnet restore Hosts/Musify.Api/Api.csproj
 
 COPY . .
 RUN dotnet publish Hosts/Musify.Api/Api.csproj -c Release -o /app --no-restore
-# Config files are AppSettings*.json (PascalCase); on case-sensitive Linux the host
-# looks for appsettings*.json. Add lowercase copies so they get loaded.
-RUN cp /app/AppSettings.json /app/appsettings.json \
-    && cp /app/AppSettings.Development.json /app/appsettings.Development.json
 
 FROM mcr.microsoft.com/dotnet/aspnet:10.0 AS runtime
 WORKDIR /app