fix(dockerfile): add fdisk package for sfdisk (partition table writing)

sfdisk is in the fdisk package on Debian bookworm, not util-linux.
util-linux provides lsblk/blkid/mount; fdisk provides sfdisk/fdisk/cfdisk.
Without this, FormatAndMount fails at the partitioning step.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

controller/Dockerfile

@@ -45,7 +45,8 @@ FROM debian:bookworm-slim
 # - sqlite3: for SQLite backup
 # - git: for stack sync from Gitea
 # - curl: for health pings and debugging
-# - util-linux: lsblk, blkid, sfdisk, mount (storage init)
+# - util-linux: lsblk, blkid, mount (storage init)
+# - fdisk: sfdisk (partition table writing — separate from util-linux on Debian)
 # - e2fsprogs: mkfs.ext4 (filesystem formatting)
 # - rsync: for data migration between storage paths
 # - parted: partprobe (partition table re-read after sfdisk)
@@ -59,6 +60,7 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     default-mysql-client \
     sqlite3 \
     util-linux \
+    fdisk \
     e2fsprogs \
     rsync \
     parted \