fix(infra): pin ocr-volume-init to alpine:3.21 and drop project network

alpine:3 is a moving tag — pinning to 3.21 makes builds reproducible and
rollbacks possible. networks: [] removes the init container from the project
network since it only needs volume access, not network access (least privilege).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

docker-compose.prod.yml

@@ -134,7 +134,7 @@ services:
   # created before the non-root ocr user was introduced in commit 1aca4c4a)
   # and guarantees /app/cache/.tmp exists for TMPDIR staging. See ADR-021.
   ocr-volume-init:
-    image: alpine:3
+    image: alpine:3.21
     command:
       - sh
       - -c
@@ -142,6 +142,7 @@ services:
     volumes:
       - ocr-models:/app/models
       - ocr-cache:/app/cache
+    networks: []
     restart: "no"
 
   ocr-service: