chore: drop unused imports and a dead constant (no behaviour change)

- backtest/diagnose_filters: unused BacktestConfig import
- pair_signals/__init__: unused PlatformConfig import
- scripts/shadow_scan: unused pandas import
- research burst_precursors / calendar_impact / duka_spread: unused sys / COST / struct
- test_all: MagicMock, StringIO, Colors and two function-local imports that
  nothing in the test bodies used
- telegram_bot: ViolentWatch is only a type hint -> imported under TYPE_CHECKING

compare_oanda_tiingo.py and oanda_td_divergence.py are left alone here because
the hygiene commit deletes them.

Suite: 464 passed, 0 failed (unchanged).

Claude-Session: https://claude.ai/code/session_0156YhogLytzj9P2iH1skGbL

backtest/research/calendar_impact.py

@@ -7,7 +7,7 @@ import argparse, sys
 from pathlib import Path
 import numpy as np, pandas as pd
 sys.path.insert(0, str(Path(__file__).resolve().parent))
-from open_trade_bursts import wilder_atr, simulate, r_of, stats, K, COST
+from open_trade_bursts import wilder_atr, simulate, r_of, stats, K
 
 CAL = Path(__file__).resolve().parent.parent / "cache" / "calendar.parquet"