fix: ignore duplicate matches whose file is gone

The hash lives in the database and the file does not, so a file deleted behind
the app's back went on refusing its own replacement. Matches are now checked
against disk, and re-adding a book's own missing file writes it back into the
row that already describes it.

backend/tests/unit/test_services/test_book_service.py

@@ -595,3 +595,48 @@ class TestBookPathCollisions:
         assert len(book.files) == 2
         for file in book.files:
             assert (Path(book.path) / file.path).is_file()
+
+
+@pytest.mark.asyncio
+class TestMissingFiles:
+    """A row whose bytes are gone must not stand in for the file itself."""
+
+    async def test_a_missing_file_is_not_a_duplicate(
+        self, books_service: BookService, test_library: m.Library
+    ) -> None:
+        """Otherwise the library refuses to take back a file it can no longer open."""
+        created = await books_service.create_many_from_files(
+            BooksCreateFromFiles(files=[upload(EPUB)]), test_library
+        )
+        book = await books_service.get(created.books[0].id)
+        (Path(book.path) / book.files[0].path).unlink()
+
+        result = await books_service.create_many_from_files(
+            BooksCreateFromFiles(files=[upload(EPUB)]), test_library
+        )
+
+        assert len(result.books) == 1
+        assert result.duplicates == []
+
+        restored = await books_service.get(result.books[0].id)
+        assert (Path(restored.path) / restored.files[0].path).is_file()
+
+    async def test_re_adding_a_missing_file_puts_it_back(
+        self, books_service: BookService, test_library: m.Library
+    ) -> None:
+        """The book already has a row for it, so the bytes go back where it says."""
+        created = await books_service.create_many_from_files(
+            BooksCreateFromFiles(files=[upload(EPUB)]), test_library
+        )
+        book_id = created.books[0].id
+
+        book = await books_service.get(book_id)
+        path = Path(book.path) / book.files[0].path
+        path.unlink()
+
+        await books_service.add_files(book_id, [upload(EPUB)], test_library)
+
+        book = await books_service.get(book_id)
+        assert len(book.files) == 1
+        assert path.is_file()
+        assert path.read_bytes() == EPUB.read_bytes()