gitea-backup-pull: switch vlda-01 retention from a fixed count to a 30-day rotation

KEEP=30 (last 30 archives) approximated 30 days only if there was
exactly one dump per day. Prune by file mtime instead (find -mtime +30)
so a catch-up burst after time offline doesn't prematurely evict
recent dumps, and a quiet stretch doesn't hoard month-old ones.

README.md

@@ -479,8 +479,14 @@ The SSH keypair needed no special handling: Unraid already auto-mirrors
   relying on the file's own (nonexistent) `+x`.
 - Lands in `/mnt/disks/WSD2L840/git/` — an **Unassigned Device** (external
   disk, not part of the protected array), not the RAM-backed root — keeps
-  the last 30 locally, now the real long-term history given alpha deletes
-  its own copies right after a verified pull.
+  a rolling 30 calendar days locally (by file `mtime`, not a fixed file
+  count — a burst of catch-up pulls after being offline a while shouldn't
+  prematurely evict genuinely recent dumps, and a quiet stretch shouldn't
+  keep month-old ones around just for lack of newer arrivals), now the
+  real long-term history given alpha deletes its own copies right after a
+  verified pull. Verified live: a synthetic 40-day-old dump + its
+  checksum file were correctly pruned while a genuine 20-day-old one and
+  everything newer were left untouched.
 - **The device isn't guaranteed to already be mounted** when the script
   runs, so it resolves the disk by filesystem label (`findfs
   LABEL=WSD2L840`, not a hardcoded `/dev/sdX` — USB/Unassigned-Device