fix: ruff findings

tests/test_collector_history_tracer.py

@@ -8,7 +8,6 @@ Seams:
 - write side: run_collection() → observation store
 - read side: get_status(), compute_projection() → observation store
 """
-import os
 import sqlite3
 from datetime import datetime, timedelta, timezone
 from pathlib import Path
@@ -19,10 +18,9 @@ import sys
 
 sys.path.insert(0, str(Path(__file__).parent.parent / "src"))
 
-from fenris.collector import run_collection, normalize_identity
-from fenris.store import init_store, get_store_path, SCHEMA_VERSION
-from fenris.monitoring_periods import ensure_period_open, close_period, get_open_period
-from fenris.day_aggregate import derive_day, derive_all_days
+from fenris.collector import run_collection
+from fenris.store import init_store, SCHEMA_VERSION
+from fenris.monitoring_periods import close_period, get_open_period
 
 
 # ---------------------------------------------------------------------------
@@ -573,7 +571,6 @@ class TestDisplayStates:
 
     def test_one_sample_awaiting_another_in_tui(self, tmp_path):
         """One sample → TUI shows awaiting state."""
-        from fenris.tui import FenrisTuiApp
         db = tmp_path / "test.db"
         conn = init_store(db)
         conn.execute(