Backend: cover cache and web API changes

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

python-backend/tests/test_library.py

@@ -820,6 +820,41 @@ class TestCoverDownscaling:
         # The untouched original still comes back, because colour extraction wants it.
         assert art == (folder / "cover.jpg").read_bytes()
 
+    def test_store_cover_also_writes_a_smaller_baseline_thumbnail(self, tmp_path: Path) -> None:
+        from PIL import Image
+
+        from musicmouse.library.cache import THUMB_COVER_PX, LibraryCache
+
+        cache = LibraryCache(tmp_path)
+        cache.prepare()
+        cache.store_cover("abc123", self._jpeg((2400, 1800)))
+        with Image.open(cache.thumb_path("abc123")) as thumb:
+            assert max(thumb.size) == THUMB_COVER_PX
+            assert thumb.format == "JPEG"
+            assert "progressive" not in thumb.info
+            assert abs(thumb.width / thumb.height - 2400 / 1800) < 0.02
+
+    def test_thumbnails_are_backfilled_for_covers_stored_without_one(
+        self, tmp_path: Path
+    ) -> None:
+        from PIL import Image
+
+        from musicmouse.library.cache import THUMB_COVER_PX, LibraryCache
+
+        cache = LibraryCache(tmp_path)
+        cache.prepare()
+        cache.cover_path("old").write_bytes(self._jpeg((384, 384)))
+        assert not cache.thumb_path("old").exists()
+
+        assert cache.shrink_stored_covers() == 0  # nothing oversized...
+        with Image.open(cache.thumb_path("old")) as thumb:  # ...but the thumb appeared
+            assert max(thumb.size) == THUMB_COVER_PX
+        # Steady state: the thumbnail is not regenerated, and is not mistaken for a cover.
+        before = cache.thumb_path("old").stat().st_mtime_ns
+        assert cache.shrink_stored_covers() == 0
+        assert cache.thumb_path("old").stat().st_mtime_ns == before
+        assert not cache.thumb_path("old.thumb").exists()
+
     def test_covers_written_by_an_older_version_are_migrated(self, tmp_path: Path) -> None:
         """The scanner reuses an unchanged album without re-reading its tags, so a
         cover stored before the limit existed would otherwise never be rewritten."""