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>

app/bracket_view.py

@@ -34,13 +34,13 @@ def build_bracket_view(year, bracket_payload, fetched_at=None):
     def slot(letter):
         return _matchup(year, letter, series_by_letter.get(letter))
 
-    east_r1 = [slot(l) for l in EAST_R1]
-    west_r1 = [slot(l) for l in WEST_R1]
-    east_r2 = [slot(l) for l in EAST_R2]
-    west_r2 = [slot(l) for l in WEST_R2]
-    east_cf = [slot(l) for l in EAST_CF]
-    west_cf = [slot(l) for l in WEST_CF]
-    cup = [slot(l) for l in CUP_FINAL]
+    east_r1 = [slot(ltr) for ltr in EAST_R1]
+    west_r1 = [slot(ltr) for ltr in WEST_R1]
+    east_r2 = [slot(ltr) for ltr in EAST_R2]
+    west_r2 = [slot(ltr) for ltr in WEST_R2]
+    east_cf = [slot(ltr) for ltr in EAST_CF]
+    west_cf = [slot(ltr) for ltr in WEST_CF]
+    cup = [slot(ltr) for ltr in CUP_FINAL]
 
     return {
         "year": year,