Fix cross-folder links leaving the original next to the new link

When a match lives in another folder than the master (e.g. Game Rips/ vs
Video Games/), linking creates the new link at the master's path and then
removes the original. That removal re-checked the original's modification
time, which FUSE filesystems such as Unraid's /mnt/user can report
inconsistently; the check then failed after the link was already made,
leaving both files and a failed operation whose quarantine copy was hidden.

- Once the original is quarantined, verify it by identity (same inode as
  the quarantine copy) instead of by timestamp. Identical files without a
  quarantine copy keep the strict check, now with a detailed error.
- If the original still cannot be removed, roll back: remove the new link
  and the unneeded quarantine copy so nothing changes. If the original was
  swapped for another file mid-way, its quarantine copy is kept.
- Linking again when the master's path already holds a hardlink of the
  master only retires the leftover original, which cleans up files left
  by the old behaviour; undo then keeps that pre-existing link
  (migration 5 records this).
- Promote gets the same identity check and rollback.

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

internal/store/store.go

@@ -114,6 +114,10 @@ var migrations = []string{
 	// matches that are not at the same path (other format, approximate, ...).
 	`ALTER TABLE operations ADD COLUMN match_kind TEXT NOT NULL DEFAULT '';
 	ALTER TABLE operations ADD COLUMN match_reason TEXT NOT NULL DEFAULT '';`,
+
+	// 5: a link whose target already existed (only the original was retired);
+	// undo must then leave that target alone.
+	`ALTER TABLE operations ADD COLUMN link_preexisted INTEGER NOT NULL DEFAULT 0;`,
 }
 
 // Open opens (and migrates) the database in dataDir.