nfs-mirror: append transferred files to offsite-sync manifest

Step 1 of offsite-sync-backup is incremental on non-monthly days,
driven by /mnt/backup/.changed-files which only daily-backup wrote
to. nfs-mirror's writes were therefore invisible to Step 1 until the
next monthly --delete pass — which would *also* wipe data
pre-positioned on Synology pve-backup/ (e.g. the in-place btrfs
rename we just did to relocate ~160G of NFS subtrees from
/Backup/Viki/nfs/<svc>/ to /Backup/Viki/pve-backup/<svc>/).

Fix: snapshot a timestamp before rsync, then after rsync use
`find -newer $STAMP -type f -printf '%P\n'` to enumerate every file
nfs-mirror created/modified and append to the manifest. Paths are
relative to /mnt/backup/ (matches Step 1 --files-from expectation).
State files are excluded.

The current in-flight first run started before this patch was
deployed, so its writes won't auto-populate the manifest — a one-off
manual backfill will be done after it completes.

stacks/blog/backend.tf

@@ -1,7 +1,7 @@
 # Generated by Terragrunt. Sig: nIlQXj57tbuaRZEa
 terraform {
   backend "pg" {
-    conn_str    = "postgres://terraform_state:ts7DGcKmTTY-5ujz4mhh@10.0.20.200:5432/terraform_state?sslmode=disable"
+    conn_str    = "postgres://terraform_state:LicuZK1nVl4ILE5HF-A9@10.0.20.200:5432/terraform_state?sslmode=disable"
     schema_name = "blog"
   }
 }