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/src/BigRegister.Api/Domain/Letters/LetterHtml.cs

@@ -6,7 +6,7 @@ using BigRegister.Api.Data;
 namespace BigRegister.Domain.Letters;
 
 /// <summary>
-/// Server-rendered letter HTML (WP-25) — the archived, "what is sent" artifact.
+/// Server-rendered letter HTML — the archived, "what is sent" artifact.
 /// Mirrors the FE letter canvas' class vocabulary exactly (<c>public/letter.css</c>,
 /// the FE⇄BE contract; LetterHtmlTests' class-parity test is the fence against drift).
 ///
@@ -151,7 +151,8 @@ public static class LetterHtml
   private static string EncLines(string s) => Enc(s).Replace("\n", "<br>");
 
   // Walks up from the running assembly's own directory (NOT the process cwd, which
-  // varies by how `dotnet run`/docker/tests invoke it — see docs/project/backlog/WP-25) until
+  // varies by how `dotnet run`/docker/tests invoke it — see the letter-preview-html design
+  // note in docs/project/backlog/) until
   // it finds `public/letter.css`. docker-compose.yml bind-mounts `./public` under the
   // api container's `/src` for exactly this walk to resolve there too.
   private static string FindLetterCss()