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). A backup uploads a blob already on the destination again
instead of skipping it, so it gets a fresh time. The rclone backend takes
each object's time from the listing (use_server_modtime).

Cost: after database delete, the next backup uploads all its data again.
Side effect: a rebuilt blob replaces the stored copy; if compression_level
or the recipients changed since, older snapshots listing it fail verify.

Model: opus-5-5

internal/cli/config.go

@@ -239,6 +239,13 @@ 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. 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