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 covers one user on one machine, so a prune from another host
or user sharing the destination deleted a running backup's blobs.

Prune now deletes an unreferenced blob only once its listed time is older
than the new config key prune_grace_period (default 24h; a negative value
is refused). A newer blob, or one listed with no time, is kept and counted
(blobs_too_new). The rclone backend takes each object's time from the
listing (use_server_modtime).

Decision: a blob a backup reuses from an earlier run that stopped before
its manifest keeps its old time and is not protected; the docs and config
comments say so.

Model: opus-5-5

internal/cli/config.go

@@ -239,6 +239,16 @@ storage_url: ""
 # Default: 3
 # compression_level: 3
 
+# 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. It does not protect a blob a backup reuses from an
+# earlier run that stopped before its manifest: that blob keeps the time of
+# its first upload, so a prune can delete it.
+# Accepts: 36h, 90m, etc. 0s deletes them at once.
+# Default: 24h
+# prune_grace_period: 24h
+
 # Hostname used in snapshot IDs. Default: system hostname.
 # hostname: myserver