fix: bootstrap IV history with 1y realized vol to unblock IV Rank from 50

IV Rank was stuck at 50 for all tickers because the formula returns 50.0
when iv_max == iv_min (not enough historical snapshots). Added:
- bootstrap_iv_history(): downloads 1y of closes per ticker, computes
  30d rolling realized vol (annualized), saves each day to iv_history
- POST /api/options-vol/bootstrap-history endpoint (runs in background)
- OptionsLab: auto-detect when IV Rank needs bootstrapping and show
  an amber banner with one-click "Initialiser historique" button

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

backend/routers/options_vol.py

@@ -142,6 +142,24 @@ def refresh_watchlist(background_tasks: BackgroundTasks):
     return {"status": "refresh started", "tickers": len(__import__("services.iv_engine", fromlist=["IV_WATCHLIST"]).IV_WATCHLIST)}
 
 
+@router.post("/bootstrap-history")
+def bootstrap_iv_history_endpoint(background_tasks: BackgroundTasks, min_existing: int = 30):
+    """
+    Bootstrap iv_history with 1 year of realized volatility for all watchlist tickers.
+    Only fills missing dates — safe to call multiple times.
+    Runs in background (takes ~30-60s for 18 tickers).
+    """
+    def _run():
+        from services.iv_engine import bootstrap_iv_history, IV_WATCHLIST
+        logger.info(f"[IV Bootstrap] Starting for {len(IV_WATCHLIST)} tickers")
+        result = bootstrap_iv_history(tickers=IV_WATCHLIST, min_existing=min_existing)
+        total = sum(v.get("inserted", 0) for v in result.values() if isinstance(v, dict))
+        logger.info(f"[IV Bootstrap] Done — {total} rows inserted across {len(result)} tickers")
+
+    background_tasks.add_task(_run)
+    return {"status": "bootstrap started", "message": "Chargement historique 1 an réalisé vol — vérifier les logs dans ~60s"}
+
+
 @router.get("/for-trade/{underlying}")
 def get_iv_for_trade(underlying: str):
     """