style: apply ruff format and fix lint issues in playoff modules

- Rename single-letter `l` loop variables in bracket_view to satisfy E741
- Drop unused `json` import from test_playoff_cache (F401/F811)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_bracket_view.py

@@ -1,8 +1,19 @@
 from app.bracket_view import build_bracket_view
 
 
-def _series(letter, top_abbrev, top_id, top_wins, bot_abbrev, bot_id, bot_wins,
-            rnd=1, winning_id=None, top_seed="D1", bot_seed="WC1"):
+def _series(
+    letter,
+    top_abbrev,
+    top_id,
+    top_wins,
+    bot_abbrev,
+    bot_id,
+    bot_wins,
+    rnd=1,
+    winning_id=None,
+    top_seed="D1",
+    bot_seed="WC1",
+):
     return {
         "seriesLetter": letter,
         "playoffRound": rnd,