fix(engine): cross-backend integrity guard with retry-then-damaged

A truncated debrid download (in-memory byte counter hit 100% while the
NFS write-back silently dropped most of the bytes) was marked completed.
The 1.1.6 fsync fix closed the debrid-specific hole; this generalizes the
guarantee so "completed" never means a corrupt file on ANY backend.

- IntegrityError + bounded retry: on a corrupt/short result the manager
  re-downloads the same source up to 3x (clean start), then surfaces the
  task as damaged ("corrupt download:" prefix) instead of completing it.
- verify (size mismatch / empty), debrid (incomplete / post-write / flush),
  torrent (BytesMissing), usenet (par2 unrepairable / repair-failed) all
  classify integrity failures so they route through the retry/damaged path.
- scanner: a file ffprobe can't read is emitted as a damaged library_item
  (reason "unreadable") instead of being silently dropped from the sync.
- tests: manager retry-then-success + retry-exhausted-then-damaged,
  verifying->resolving transition, damaged sync item.

internal/engine/verify.go

@@ -29,14 +29,19 @@ func verify(result *Result) error {
 	}
 
 	if actualSize == 0 {
-		return fmt.Errorf("download is empty: %s", result.FilePath)
+		// Integrity, not transport: a zero-byte result is corrupt — let the manager
+		// re-download clean rather than surface an empty file as completed.
+		return integrityErr("empty", "download is empty: %s", result.FilePath)
 	}
 
-	// If we know the expected size, check within 2% tolerance
+	// If we know the expected size, check within 2% tolerance (container/muxing
+	// overhead). A shortfall beyond that is a truncated/corrupt file — classify it
+	// as an IntegrityError so the manager re-downloads clean instead of completing
+	// a half file (the last line of defense across every backend).
 	if result.Size > 0 {
 		tolerance := int64(float64(result.Size) * 0.02)
 		if actualSize < result.Size-tolerance {
-			return fmt.Errorf("size mismatch: expected %d, got %d", result.Size, actualSize)
+			return integrityErr("size_mismatch", "size mismatch: expected %d, got %d", result.Size, actualSize)
 		}
 	}