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/README.md

@@ -15,7 +15,7 @@ status codes and error envelope are production-shaped.
 covers it, see `docker-compose.yml`) does **not** lose data. Delete the file to
 reset demo data back to empty, the same state a fresh clone starts from. This is
 a deliberate, right-sized choice for a POC (SQLite, no external DB service) — see
-`docs/project/backlog/WP-22-durable-persistence.md`.
+the durable-persistence design note in `docs/project/backlog/`.
 
 ## Run
 
@@ -76,7 +76,7 @@ semantics) is introduced as **`/api/v2`** served alongside v1 until clients migr
 - `Diplomas/DiplomaRules.cs` — profession derivation + which policy questions apply.
 - `Registrations/HerregistratieRule.cs` — eligibility + reason + status invariant.
 - `Intake/IntakePolicy.cs` — scholing threshold + completeness re-validation on submit
-  (`RejectIncompleteScholing`, WP-69).
+  (`RejectIncompleteScholing`).
 - `Submissions/SubmissionRules.cs` — submit rejections + reference generation.
 
 ## Typed client (NSwag)