fix: downgrade pdf-parse to 1.1.1 to eliminate native dependency

pdf-parse@2.4.5 depends on @napi-rs/canvas (native Skia binary compiled
with AVX instructions) which crashes with SIGILL on the Synology NAS CPU.
Version 1.1.1 is pure JavaScript and works on any architecture.

Dockerfile.backend

@@ -27,10 +27,6 @@ FROM node:20-alpine AS runner
 WORKDIR /app
 ENV NODE_ENV=production
 
-# @napi-rs/canvas (dep of pdf-parse) ships a musl pre-built binary that
-# needs these compatibility / font libraries at runtime.
-RUN apk add --no-cache libc6-compat fontconfig freetype
-
 COPY --from=build /app/backend/dist ./dist
 COPY --from=build /app/backend/package.json ./package.json
 COPY --from=build /app/node_modules ./node_modules