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/tests/BigRegister.Tests/TestWebApplicationFactory.cs

@@ -1,8 +1,8 @@
 using Microsoft.AspNetCore.Hosting;
 using Microsoft.AspNetCore.Mvc.Testing;
 
-// WP-22's stores read a single static Db.ConnectionString (there's no DI, matching
-// their pre-WP-22 static-Dictionary shape — see Data/Db.cs). That's correct for a
+// These stores read a single static Db.ConnectionString (there's no DI, matching
+// their earlier static-Dictionary shape — see Data/Db.cs). That's correct for a
 // real single-instance process, but xUnit's default parallel-across-classes
 // execution would run multiple WebApplicationFactory hosts concurrently in this
 // ONE test process, each overwriting that same static field with its own temp-file
@@ -14,7 +14,7 @@ using Microsoft.AspNetCore.Mvc.Testing;
 namespace BigRegister.Tests;
 
 /// <summary>
-/// WP-22 moved Applications/Documents/Briefs off in-memory dictionaries onto a real
+/// A migration moved Applications/Documents/Briefs off in-memory dictionaries onto a real
 /// SQLite file (see Data/Db.cs). Unlike static dictionaries, a shared file path
 /// would let concurrent test classes' WebApplicationFactory instances hit the same
 /// file at once — xUnit runs different test classes in parallel by default, and
@@ -29,7 +29,7 @@ public sealed class TestWebApplicationFactory : WebApplicationFactory<Program>
 
   protected override void ConfigureWebHost(IWebHostBuilder builder) => builder
       .UseSetting("ConnectionStrings:AppDb", $"Data Source={_dbPath}")
-      // WP-52: a fixed shared secret so NotificatieTests can exercise the accept path —
+      // A fixed shared secret so NotificatieTests can exercise the accept path —
       // the appsettings.json default is "" (reject everything), which no test should rely on.
       .UseSetting("Zgw:NotificatieAuthorization", "test-nrc-secret");