fix: a release its download client will not let Minstrel follow is held back, not taken every pass (#6176)

The Black Keys' Ohio Players was grabbed 40 times in a day. FabledExchange
already held the torrent under another category from before Minstrel took
the downloads over: the add answered "duplicate", which counted as added,
and Exchange's listing, filtered to Minstrel's category, never showed it.
Half an hour later the grab ended gone, and the next pass took it again.

- A duplicate add that the client does not list for Minstrel is not a
  grab: the release is held back for a day with the client's own reason
  (already downloading X (category)), and the search moves on. Forced,
  it is refused with that reason.
- A release that ends gone a second time within a week is held back for
  a day, whatever the cause.
- release_blocks gains the reason 'gone' (migration 0098), shown as
  "Held back: its download client lost track of it".

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

internal/acquire/memory.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"time"
 
 	"github.com/jackc/pgx/v5"
 	"github.com/jackc/pgx/v5/pgtype"
@@ -24,8 +25,17 @@ const (
 	// BlockStalled: given up on because it would not finish (#5523); held
 	// back until the block's expires_at.
 	BlockStalled = "stalled"
+	// BlockGone: its download client would not let Minstrel follow it, held
+	// outside Minstrel's category or leaving the client twice (#6176); held
+	// back for GoneBlock.
+	BlockGone = "gone"
 )
 
+// GoneBlock is how long a release its client lost track of is held back:
+// long enough that a release which keeps vanishing is tried once a day rather
+// than on every pass, short enough that it is taken once the client lets go.
+const GoneBlock = 24 * time.Hour
+
 // Grab origins, matching grabs_origin_check.
 const (
 	OriginSchedule = "schedule"
@@ -106,6 +116,11 @@ func BlockSentence(reason, detail string) string {
 			return "Refused on import: " + detail
 		}
 		return "Refused on import: its files were not the album."
+	case BlockGone:
+		if detail != "" {
+			return "Held back for now: " + detail
+		}
+		return "Held back for now: its download client lost track of it."
 	case BlockStalled:
 		if detail != "" {
 			return "Given up on for now: " + detail