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/duka_spread.py

@@ -10,7 +10,6 @@ bi5 format: LZMA 'alone' stream of 20-byte big-endian records
 XAUUSD prices scaled x1000. URL months are 0-indexed.
 """
 import lzma
-import struct
 import sys
 from concurrent.futures import ThreadPoolExecutor
 from datetime import date, timedelta