fix: return the publisher an EPUB declares

The lookup discarded its own result and fell off the end of the function, so
no EPUB ever contributed a publisher.

backend/tests/unit/test_metadata_extractor.py

@@ -1,4 +1,5 @@
 import pytest
+from ebooklib import epub
 from pathlib import Path
 from datetime import date
 from chitai.services.metadata_extractor import (
@@ -172,3 +173,20 @@ class TestEditionFromFiles:
 
         assert metadata["title"] == "The Project Gutenberg eBook #33283: Calculus Made Easy"
         assert metadata["edition"] == 2
+
+
+class TestEpubPublisher:
+    """The publisher was looked up and then dropped on the floor."""
+
+    def test_a_declared_publisher_is_returned(self) -> None:
+        """
+        The lookup discarded its own result and fell off the end of the function, so
+        every EPUB reported no publisher no matter what it said.
+        """
+        book = epub.EpubBook()
+        book.add_metadata("DC", "publisher", "No Starch Press")
+
+        assert EpubExtractor._extract_publisher(book) == "No Starch Press"
+
+    def test_no_publisher_is_none(self) -> None:
+        assert EpubExtractor._extract_publisher(epub.EpubBook()) is None