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/unit/test_filesystem_library.py

@@ -2,7 +2,10 @@
 
 from pathlib import Path
 
-from chitai.services.filesystem_library import BookPathGenerator, sanitize_path_component
+from chitai.services.filesystem_library import (
+    BookPathGenerator,
+    sanitize_path_component,
+)
 
 
 ROOT = Path("/library")
@@ -23,12 +26,15 @@ def test_a_book_with_no_authors() -> None:
 
 
 def test_a_series_adds_a_level_and_pads_the_position() -> None:
-    assert path_for(
-        title="Persepolis Rising",
-        authors=["James S. A. Corey"],
-        series="The Expanse",
-        series_position="7",
-    ) == ROOT / "James S. A. Corey" / "The Expanse" / "07 - Persepolis Rising"
+    assert (
+        path_for(
+            title="Persepolis Rising",
+            authors=["James S. A. Corey"],
+            series="The Expanse",
+            series_position="7",
+        )
+        == ROOT / "James S. A. Corey" / "The Expanse" / "07 - Persepolis Rising"
+    )
 
 
 def test_a_slash_in_a_title_does_not_add_a_directory() -> None:
@@ -43,16 +49,25 @@ def test_a_slash_in_a_title_does_not_add_a_directory() -> None:
     generated = path_for(title="Back in Black: AC/DC", authors=["Murray Engleheart"])
 
     assert generated == ROOT / "Murray Engleheart" / "Back in Black: AC_DC"
-    assert generated.relative_to(ROOT).parts == ("Murray Engleheart", "Back in Black: AC_DC")
+    assert generated.relative_to(ROOT).parts == (
+        "Murray Engleheart",
+        "Back in Black: AC_DC",
+    )
 
 
 def test_a_slash_in_an_author_or_series_is_handled_too() -> None:
     assert path_for(title="Split", authors=["A/B Collective"]) == (
         ROOT / "A_B Collective" / "Split"
     )
-    assert path_for(
-        title="Volume One", authors=["Someone"], series="Either/Or", series_position="1"
-    ) == ROOT / "Someone" / "Either_Or" / "01 - Volume One"
+    assert (
+        path_for(
+            title="Volume One",
+            authors=["Someone"],
+            series="Either/Or",
+            series_position="1",
+        )
+        == ROOT / "Someone" / "Either_Or" / "01 - Volume One"
+    )
 
 
 def test_control_characters_are_removed() -> None: