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/Data/Db.cs

@@ -6,7 +6,7 @@ namespace BigRegister.Api.Data;
 /// <summary>
 /// Factory for short-lived <see cref="AppDbContext"/> instances. The three stores
 /// (ApplicationStore/DocumentStore/BriefStore) are static classes — that shape
-/// predates WP-22 and this WP keeps it — so they can't take a constructor-injected
+/// predates this persistence layer, which keeps that shape — so they can't take a constructor-injected
 /// DbContext; each store method opens one here, uses it, and disposes it under its
 /// own lock instead.
 /// </summary>