fix: resolve critical/important bugs from code review (#1,#2,#4,#5,#6,#8)

- #1: OI division by zero — already fixed (prev_oi == 0.0 guard exists)
- #2: cumulative trade count used max() instead of sum(), breaking ML trigger
- #4: fetch_history API calls now retry 3x with exponential backoff
- #5: parquet upsert now deduplicates timestamps before sort
- #6: record_pnl() is now async with Lock for multi-symbol safety
- #8: exit_price=0.0 skips close handling with warning log

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

scripts/weekly_report.py

@@ -418,7 +418,7 @@ def generate_report(
         for rpath in sorted(rdir.glob("report_*.json")):
             try:
                 prev = json.loads(rpath.read_text())
-                cumulative = max(cumulative, prev.get("live_trades", {}).get("count", 0))
+                cumulative += prev.get("live_trades", {}).get("count", 0)
             except (json.JSONDecodeError, KeyError):
                 pass