Keep unreferenced blobs newer than prune_grace_period (closes #387)

A backup uploads its blobs before the manifest that lists them, and the
process lock is per user per machine, so a prune from another host or
user sharing the destination deleted a running backup's blobs; that
backup still reported success and could not be restored.

Prune now deletes an unreferenced blob only once it is older than the new
config key prune_grace_period (default 24h), and reports how many it kept
(blobs_too_new under --json). The storage listing now carries each
object's modification time. remote nuke is unchanged. The README says what
the lock covers and that a shared destination relies on the grace period
or needs one prefix per host.

Model: opus-5-5

config.example.yml

@@ -328,6 +328,15 @@ storage_url: "rclone://myremote/path/to/backups"
 # Higher = better compression but slower
 # Default: 3
 compression_level: 5
+
+# How old a blob that no snapshot lists must be before prune deletes it
+# A backup uploads its blobs before the manifest that lists them, so on a
+# destination shared between hosts or users this must be longer than the
+# longest backup run
+# Supports: 36h, 90m, etc.; 0s deletes them at once
+# Default: 24h
+#prune_grace_period: 24h
+
 # Hostname to use in backup metadata
 # Default: system hostname
 #hostname: myserver