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).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0156YhogLytzj9P2iH1skGbL

backtest/research/burst_precursors.py

@@ -9,7 +9,7 @@ within/outside the scheduled-data hours, and an out-of-sample logistic model
 
     python backtest/research/burst_precursors.py PARQUET BAR_MINUTES [--k 3] [--start ... --end ...]
 """
-import argparse, sys
+import argparse
 import numpy as np, pandas as pd