fix(bib): import_books skips attach_file entirely for a duplicate attachment

attach_file is idempotent by filename, but a re-run still called it
for an attachment already on file — a wasted copy every time. Skip the
call outright once the dup check finds it, instead of calling it and
only using the dup flag to decide whether to count it as new.

src/bib/zotero_import.py

@@ -241,8 +241,8 @@ def import_books(
                     )
                     .fetchone()
                 )
-                store.attach_file(key, att_path)
                 if dup is None:
+                    store.attach_file(key, att_path)
                     n_new += 1
         stats["attached"] += n_new
         stats["missing"] += n_missing

tests/bib/test_zotero_import.py

@@ -208,6 +208,27 @@ class TestImportBooks:
         )
         assert len(rows) == 1  # no duplicate attachment
 
+    def test_rerun_never_calls_attach_file_for_a_duplicate(self, store, storage_dir):
+        """Minor: attach_file is idempotent-by-filename, but a re-run
+        used to call it anyway (a wasted copy) — it must now be skipped
+        entirely once the dup check finds the attachment already there."""
+        book = self._cpt_book(storage_dir)
+        tags = ["source:ama"]
+        import_books(store, [book], tags=tags)
+
+        calls = []
+        orig_attach_file = store.attach_file
+        store.attach_file = lambda *a, **k: (
+            calls.append((a, k)) or orig_attach_file(*a, **k)
+        )
+        try:
+            second = import_books(store, [book], tags=tags)
+        finally:
+            store.attach_file = orig_attach_file
+
+        assert calls == []
+        assert second["attached"] == 0
+
     def test_dry_run_makes_no_writes(self, store, storage_dir):
         book = self._cpt_book(storage_dir)
         stats = import_books(store, [book], tags=["source:ama"], dry_run=True)