chore(images): pin service image updates

Replace floating service images with reviewable tags or digests.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

converters/compose.yaml

@@ -1,7 +1,7 @@
 services:
   convertx:
     container_name: convertx
-    image: ghcr.io/c4illin/convertx:latest
+    image: ghcr.io/c4illin/convertx:v0.18.0
     restart: unless-stopped
     ports:
       - "9992:3000"
@@ -42,7 +42,7 @@ services:
 
   bentopdf:
     container_name: bentopdf
-    image: bentopdf/bentopdf:latest
+    image: bentopdf/bentopdf@sha256:4eb4ec8f5030faf87c29a73d3d5a2781f28a597cf440c3ab111eb96aee550871
     restart: unless-stopped
     labels:
       - "traefik.enable=true"