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/Domain/BeoordelingRuleTests.cs

@@ -16,7 +16,7 @@ public class BeoordelingRuleTests
   public void Only_open_statuses_are_decidable(AanvraagStatusTag tag, bool expected) =>
       Assert.Equal(expected, BeoordelingRules.CanDecide(tag));
 
-  // WP-68 (F6): the toelichting rule, moved here from an inline endpoint check.
+  // The toelichting rule, moved here from an inline endpoint check.
   [Theory]
   [InlineData(Besluit.Goedkeuren, false)]
   [InlineData(Besluit.Afwijzen, true)]
@@ -24,12 +24,12 @@ public class BeoordelingRuleTests
   public void Only_a_non_approval_requires_a_toelichting(Besluit besluit, bool expected) =>
       Assert.Equal(expected, BeoordelingRules.RequiresToelichting(besluit));
 
-  // WP-68 (T3): the transition table at the AGGREGATE level, not just against a bare tag —
+  // The transition table at the AGGREGATE level, not just against a bare tag —
   // an Aanvraag whose BesluitStatus already records a terminal decision computes a terminal
   // StatusAt, and CanDecide refuses a further besluit regardless of which one. Pins the
   // domain statement "Afgewezen/Goedgekeurd → no further besluit" independent of the
   // endpoint's own (integration-level) Already_decided_case_rejects_a_further_besluit.
-  // WP-70: built via Given, not a hand-rolled Aanvraag literal — Decided(Besluit.Afwijzen) with
+  // Built via Given, not a hand-rolled Aanvraag literal — Decided(Besluit.Afwijzen) with
   // no toelichting simply couldn't compile as a fixture here.
   [Theory]
   [InlineData(Besluit.Goedkeuren)]