feat(fp): WP-25 — server-rendered letter HTML preview

Adds LetterHtml.Render, a pure composer mirroring the FE letter canvas'
class vocabulary, behind two ExcludeFromDescription()'d endpoints
(GET /brief/preview, GET /admin/org-template/{subOrgId}/preview).
Auto-resolvable placeholders pull from seed/case data; unresolved
manual ones render as "[NOG IN TE VULLEN: label]". A sent brief
archives its composed HTML (BriefEntity.ArchivedHtml) so a later
org-template republish never changes it. FE gets a hand-written fetch
adapter (text/html, not JSON) and a "Voorbeeld" button that opens the
preview in a new tab.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docker-compose.yml

@@ -16,6 +16,11 @@ services:
       - ./backend:/src:z
       - api-bin:/src/src/BigRegister.Api/bin
       - api-obj:/src/src/BigRegister.Api/obj
+      # WP-25: LetterHtml.Render inlines public/letter.css (the FE⇄BE letter
+      # contract, repo-root sibling of backend/) — mounted here so the same
+      # walk-up-from-the-assembly lookup that works for `dotnet run`/tests also
+      # resolves inside this container, whose bind mount otherwise only sees backend/.
+      - ./public:/src/public:z
     ports:
       - '5000:5000'