Serve album covers from the service, show and sort by dates

Covers are stored with the assignment and served by the service, since Music Assistant's plain-http image URLs are blocked on an https page. Search results load covers through a signed /covers proxy. Adds a fetch-covers command for imported assignments, a migration for existing databases, and sorting assignments by artist, recently assigned or recently scanned, with both dates shown.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/conftest.py

@@ -13,11 +13,23 @@ AUTH = {"Authorization": f"Bearer {TOKEN}"}
 class FakeLibrary:
     def __init__(self):
         self.queries = []
+        self.fetched = []
+        self.images = {"http://img/1": (b"\x89PNG-lateralus", "image/png")}
 
     def search_albums(self, query, limit=12):
         self.queries.append(query)
         return [Album(artist="Tool", album="Lateralus", image="http://img/1")]
 
+    def find_album(self, artist, album):
+        found = self.search_albums(album)[0]
+        return found if (found.artist, found.album) == (artist, album) else None
+
+    def fetch_image(self, url):
+        self.fetched.append(url)
+        if url not in self.images:
+            raise ValueError("no such image")
+        return self.images[url]
+
 
 @pytest.fixture
 def store(tmp_path):