refactor: strip WP-/RB- ticket refs from backend (RD-19)

The backend half of the sweep RD-18 did for the front end. git blame
holds the provenance and stays correct when the code moves; the
comment names a closed ticket and tells the reader nothing the
sentence around it does not.

public/letter.css and LetterHtml.golden.html change together, because
the renderer inlines the CSS and the golden file snapshots the
result.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/Dockerfile

@@ -1,4 +1,4 @@
-# WP-30: lean deployable image (optional — not used by the dev demo, which keeps the SDK
+# Lean deployable image (optional — not used by the dev demo, which keeps the SDK
 # image in the root docker-compose.yml for `dotnet run` hot-reload). Build from the repo
 # root: `docker build -f backend/Dockerfile .`
 FROM mcr.microsoft.com/dotnet/sdk:10.0 AS build
@@ -9,10 +9,10 @@ RUN dotnet publish backend/src/BigRegister.Api -c Release -o /app
 FROM mcr.microsoft.com/dotnet/aspnet:10.0
 WORKDIR /app
 COPY --chown=$APP_UID:$APP_UID --from=build /app .
-# LetterHtml.Render (WP-25) walks up from AppContext.BaseDirectory looking for a sibling
+# LetterHtml.Render walks up from AppContext.BaseDirectory looking for a sibling
 # public/letter.css (the FE⇄BE letter contract) — this keeps that lookup working here too.
 COPY --chown=$APP_UID:$APP_UID public ./public
 # $APP_UID (uid/gid 1654, "app") is baked into this base image for exactly this purpose —
-# non-root, and chown'd above so it can still create/write bigregister.db (WP-22) at /app.
+# non-root, and chown'd above so it can still create/write bigregister.db at /app.
 USER $APP_UID
 ENTRYPOINT ["dotnet", "BigRegister.Api.dll"]