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

pair_signals/__init__.py

@@ -10,7 +10,7 @@ then register it in this file.
 
 from typing import Optional, List, Dict, Callable
 
-from config import PlatformConfig, Timeframe
+from config import Timeframe
 from signal_aggregator import SignalAggregator, SignalReport
 from providers import BaseDataProvider