feat(accounting): rasterise inbound PDFs server-side (never serve raw to browser)

Security hardening for inbound supplier-invoice previews. The admin UI no
longer renders raw PDFs — a malicious inbound PDF could otherwise run embedded
JS or phone home in the admin's session. Instead PDFs are rasterised to flat
PNGs server-side and only those images are shown.

- backend: new rasterizeService shells out to poppler `pdftoppm` (added to the
  Docker image via apk poppler-utils — an OS package, NOT a Node PDF lib, so it
  respects the pdfkit+pdf-lib "no third PDF lib" rule). pdftoppm executes no JS
  and fetches no remote resources, so it doubles as the SSRF/phone-home guard.
  Rendered pages cached under storage/business-docs/inbound/rendered/<id>/.
  - GET /inbound/:id/page/:n streams the rasterised PNG (CSP default-src 'none'
    + nosniff). GET /inbound/:id/file now serves PDFs as a DOWNLOAD only
    (Content-Disposition: attachment) — never inline; images still inline.
- frontend: triage preview switched from a raw-PDF <iframe> to rasterised page
  images (getInboundPageBlob), defaulting to the LAST page (QR-bill) with
  prev/next nav for multi-page PDFs; images stream as before.
- i18n: previewError / prevPage / nextPage / pageOf (EN + DE).

REQUIRES A BACKEND IMAGE REBUILD (Dockerfile adds poppler-utils) — a plain
`docker compose pull` of a stale image won't have pdftoppm; the route then
returns 503 RASTERIZER_UNAVAILABLE and the UI shows "preview unavailable".

Verified: node -c, a pdfkit->pdftoppm rasterise smoke test (renders + caches),
en/de JSON valid, npm run build green.

backend/Dockerfile

@@ -50,8 +50,13 @@ RUN npm install -g npm@10
 # Unicode fallback; picpeak's own brand fonts (assets/fonts/, the same files
 # PDFKit + the web UI use) are registered with fontconfig further down so the
 # logo's text renders in its actual typeface, not a fallback.
+# poppler-utils provides `pdftoppm`, used to rasterise inbound supplier-invoice
+# PDFs to flat PNGs server-side so the admin UI NEVER renders a raw (possibly
+# malicious) PDF. pdftoppm does not execute embedded JS or fetch remote
+# resources, so it doubles as the SSRF/phone-home guard for untrusted inbound
+# documents (see docs/accounting-inbound-invoices.md).
 RUN apk add --no-cache dumb-init postgresql-client ffmpeg su-exec \
-    fontconfig ttf-dejavu ttf-liberation && \
+    fontconfig ttf-dejavu ttf-liberation poppler-utils && \
     fc-cache -f
 
 # Create non-root user