chore: bring the test suite up to ruff's standards

Mostly automatic: empty f-strings, unused imports, formatting. The manual half
was duplicated import blocks stranded mid-file and `== None` assertions, which
become `is None` here because they compare plain attributes -- unlike the
identical rule in filters/book.py, where they build SQL.

Two unused variables are the tests never checking the disk in either
delete_files direction. Left in place under a noqa so the gap stays visible.

backend/tests/integration/test_file_hash.py

@@ -8,7 +8,9 @@ from pathlib import Path
 # Known KOReader hashes for test files
 TEST_FILES = {
     "Moby Dick; Or, The Whale - Herman Melville.epub": {
-        "path": Path("tests/data_files/Moby Dick; Or, The Whale - Herman Melville.epub"),
+        "path": Path(
+            "tests/data_files/Moby Dick; Or, The Whale - Herman Melville.epub"
+        ),
         "hash": "ceeef909ec65653ba77e1380dff998fb",
         "content_type": "application/epub+zip",
     },
@@ -59,7 +61,9 @@ async def test_add_file_to_book_generates_correct_hash(
     first_book = TEST_FILES["Moby Dick; Or, The Whale - Herman Melville.epub"]
     first_content = first_book["path"].read_bytes()
 
-    files = [("files", (first_book["path"].name, first_content, first_book["content_type"]))]
+    files = [
+        ("files", (first_book["path"].name, first_content, first_book["content_type"]))
+    ]
     data = {"library_id": "1"}
 
     create_response = await authenticated_client.post(
@@ -75,7 +79,12 @@ async def test_add_file_to_book_generates_correct_hash(
     second_book = TEST_FILES["Calculus Made Easy - Silvanus Thompson.pdf"]
     second_content = second_book["path"].read_bytes()
 
-    add_files = [("data", (second_book["path"].name, second_content, second_book["content_type"]))]
+    add_files = [
+        (
+            "data",
+            (second_book["path"].name, second_content, second_book["content_type"]),
+        )
+    ]
 
     add_response = await authenticated_client.post(
         f"/books/{book_id}/files",