fix(maintenance): never remove the kept copy; move records before unlink

Final review of #313:

- C1: the duplicate find tells files apart by (st_dev, st_ino), so a
  source listed twice, a symlinked source inside another and a hardlink
  are one file. The preview and the job refuse a removal that
  os.path.samestat matches with its kept copy.
- I1: each file's records move to its verified kept copy in one short
  transaction before that file is unlinked; a failed move deletes
  nothing and a failed unlink leaves the records on the identical kept
  copy. The status line counts files removed and not removed.
- I2: links are looked up under every spelling of the source (the real
  path, the configured and monitored folders, ComfyUI's output).
- I5: image tags join the kept copy as their union and the removed
  copy's pending, undecided Needs review items are settled; the plan
  says so.
- I3: the VACUUM disk check measures SQLite's real temporary folder.
- M1: the token is looked at again with no await before the job starts.
- M2: the backup decision is made once and handed to the job.
- M3: backups are linked into place; startup sweeps interrupted temps.
- M4: Optimize runs alone (new "exclusive" admission) and its plan says
  the database is locked while VACUUM runs.
- M5: the reviewed duplicate merge is strict: collisions and Library
  titles are protected, and errors roll back. The legacy cleanup is
  unchanged.
- M7: 2,000 keep entries within a 1 MiB body. M8: event waits in tests.
- M9: the preview binds files by size, mtime and inode without hashing;
  the job hashes right before each unlink.

py/api/__init__.py

@@ -360,6 +360,16 @@ class PromptManagerAPI(
         # forgets them (py/api/maintenance.py).
         self._maintenance_plans = PlanStore()
 
+        # A safety backup interrupted by a crash leaves its temp copy behind.
+        try:
+            from .maintenance import sweep_safety_temps
+
+            swept = sweep_safety_temps(self.db.model.db_path)
+            if swept:
+                self.logger.info(f"Removed {len(swept)} interrupted backup copies")
+        except Exception as e:
+            self.logger.warning(f"Could not sweep interrupted backup copies: {e}")
+
         # Run cleanup on initialization to remove any existing duplicates
         try:
             removed = self.db.cleanup_duplicates()