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/Builders/AanvraagBuilder.cs

@@ -14,9 +14,9 @@ public static class TestIdentities
 }
 
 /// <summary>
-/// Type-state test-data builder for <see cref="Aanvraag"/> (WP-70; simplified at WP-73). "Build
+/// Type-state test-data builder for <see cref="Aanvraag"/>. "Build
 /// test data through the same door production code uses" — <see cref="Aanvraag"/> itself is now
-/// the closed Concept/Submitted/Decided union WP-73 introduced, so this builder no longer needs
+/// the closed Concept/Submitted/Decided union, so this builder no longer needs
 /// to mirror production's guards (step-index bounds, "Afwijzen needs a toelichting") by hand —
 /// it just calls the real nested constructors/required members, which enforce them. A call that
 /// would build an illegal Aanvraag (e.g. deciding a still-Concept aanvraag, or an Afwijzen with
@@ -61,7 +61,7 @@ public sealed class ConceptAanvraag
 
   /// Submits the draft — always assigns a Referentie AND SubmittedAt together (mirrors
   /// <c>ApplicationStore.Submit</c>), so a fixture built this way can never hit the
-  /// null-forgiving derefs the pre-WP-73 flat Aanvraag needed (there's nothing to force any
+  /// null-forgiving derefs the earlier flat Aanvraag needed (there's nothing to force any
   /// more: both are required, non-null members of <see cref="Aanvraag.Submitted"/>).
   public SubmittedAanvraag Submitted(bool autoApprovable = false) => new(_type, _owner, autoApprovable);
 
@@ -110,7 +110,7 @@ public sealed class SubmittedAanvraag
     return this;
   }
 
-  /// <summary>Records a behandelaar's decision. Unlike the pre-WP-73 builder, there is no
+  /// <summary>Records a behandelaar's decision. Unlike the earlier builder, there is no
   /// hand-written toelichting guard mirroring <c>BeoordelingRules.RequiresToelichting</c> any
   /// more — <see cref="Aanvraag.Decided.Afgewezen"/>/<see cref="Aanvraag.Decided.MeerInfoGevraagd"/>
   /// simply have a `required string Toelichting` member; the null-coalescing throw below is the
@@ -186,7 +186,7 @@ public sealed class SubmittedAanvraag
 /// (<see cref="BigRegister.Domain.Beoordeling.BeoordelingRules.CanDecide"/>); a fixture that
 /// needs a SECOND besluit (the MeerInfoGevraagd "still decidable" case) builds fresh from
 /// <see cref="Given.Concept"/> again, exactly as a real second request would. Just a one-line
-/// wrapper around the already-fully-built <see cref="Aanvraag.Decided"/> value — WP-73 moved
+/// wrapper around the already-fully-built <see cref="Aanvraag.Decided"/> value — construction moved
 /// all the actual construction (and its invariant enforcement) into
 /// <see cref="SubmittedAanvraag.Decided"/> itself, so there's nothing left for this type to do
 /// except keep <c>.Decided(...).Build()</c> a valid two-call chain for the existing test