fix(stacks): fail loudly when restic prune fails

A stale restic lock made 'forget --prune' fail silently every night
(only a warning), so the repo was never pruned. Now a prune failure is
logged as an error, sends a red notification and exits non-zero.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

bin/stacks

@@ -650,8 +650,11 @@ _backup_run() {
 
   # --- Prune ---
   log "Pruning old snapshots"
-  restic forget --prune --keep-daily 7 --keep-weekly 4 --keep-monthly 6 2>&1 \
-    || warn "restic forget failed"
+  local prune_failed=false
+  if ! restic forget --prune --keep-daily 7 --keep-weekly 4 --keep-monthly 6 2>&1; then
+    err "restic forget/prune failed (stale lock? try: restic unlock)"
+    prune_failed=true
+  fi
 
   # --- Summary ---
   local snapshot_info repo_stats summary
@@ -664,6 +667,13 @@ _backup_run() {
   [[ -n "$snapshot_info" ]]      && summary+="\nLatest: \`$snapshot_info\`"
   [[ -n "$repo_stats" ]]         && summary+="\nRepo: $repo_stats"
 
+  if $prune_failed; then
+    summary+="\n❌ **Prune FAILED** — repo is growing unpruned (stale lock?). Check \`journalctl -u stacks-backup\`."
+    _notify "#cc0000" "$summary"
+    err "Backup saved, but prune failed"
+    return 1
+  fi
+
   _notify "#36a64f" "$summary"
   log "Backup complete"
 }