Scanner: spike = current ATM IV >= +30% above recent baseline

Replaces the IV/HV >= 1.5 heuristic with a baseline-relative jump:
a symbol is flagged SPIKE when current ATM IV is at least 30%
above its recent baseline (e.g., 10% -> 13%+). Baseline prefers
the 30-day average of saved snapshots; falls back to HV30 when
no scan history exists. Each scan persists a snapshot so the
baseline self-improves over time. BIG MOVE (|chg%| >= 3%) is now
shown as a separate badge instead of a spike requirement.

- snapshots.ts: add getAverageAtmIv(symbol, days)
- datafetch.ts: save snapshot per scan; compute baselineIv,
  baselineSrc, baselineN, ivJumpPct; spike from jump threshold
- options.ts: /api/scan returns new baseline + jump fields
- scanner.html: header copy, table columns (Baseline IV, IV Δ%),
  default sort by ivJumpPct desc, separate SPIKE/BIG MOVE badges

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

backend/src/lib/datafetch.ts

@@ -2,6 +2,8 @@ import YahooFinance from "yahoo-finance2";
 import type { OptionsResult, CallOrPut } from "yahoo-finance2/modules/options";
 import { bsPrice, bsGreeks, impliedVol } from "./blackscholes.js";
 import type { OptionQuote, ChainSnapshot } from "./analytics.js";
+import { computeSkewMetrics } from "./analytics.js";
+import { saveSnapshot, getAverageAtmIv } from "../db/snapshots.js";
 import {
   fmpEnabled,
   fmpExpirations,
@@ -9,6 +11,10 @@ import {
   fmpQuote,
 } from "./fmp.js";
 
+/** Spike threshold — current ATM IV must be at least this much above the baseline. */
+const SPIKE_JUMP_PCT = 0.30;       // +30%
+const SPIKE_BIG_MOVE_PCT = 3;      // |today Δ| ≥ 3%
+
 const yf = new YahooFinance({ suppressNotices: ["yahooSurvey"] });
 
 const RISK_FREE_RATE = 0.05;
@@ -294,8 +300,13 @@ export type ScanResult = {
   changePct: number;
   atmIv: number;
   hv30: number;
-  ivHv: number;       // atmIv / hv30
-  spike: boolean;     // true if atmIv/hv30 > 1.5 or |changePct| >= 3
+  baselineIv: number;     // recent-30d avg ATM IV from snapshots, else HV30
+  baselineSrc: "history" | "hv30" | "none";
+  baselineN: number;      // # of snapshot rows feeding the baseline (0 = HV30 fallback)
+  ivJumpPct: number;      // (atmIv - baselineIv) / baselineIv
+  ivHv: number;           // atmIv / hv30 (informational; pre-baseline metric)
+  spike: boolean;         // ivJumpPct ≥ 30%
+  bigMove: boolean;       // |today Δ| ≥ 3%
   expiry: string;
   error?: string;
 };
@@ -361,7 +372,33 @@ export async function scanSymbol(symbol: string): Promise<ScanResult> {
 
   const hv30 = await fetchHistoricalVol(symbol);
   const ivHv = hv30 > 0 ? atmIv / hv30 : 0;
-  const spike = ivHv >= 1.5 || Math.abs(changePct) >= 3;
 
-  return { symbol, spot, change, changePct, atmIv, hv30, ivHv, spike, expiry };
+  // Save this scan as a snapshot so the baseline grows over time.
+  // Use the snapshot's chain so per-strike IV is recorded too.
+  try {
+    if (atmIv > 0 && spot > 0) {
+      const snap = {
+        symbol, expiry, spot, spotIv: hv30,
+        timestamp: new Date().toISOString(),
+        chain,
+      };
+      saveSnapshot(symbol, expiry, spot, computeSkewMetrics(snap), chain);
+    }
+  } catch { /* non-fatal */ }
+
+  // Baseline IV: prefer 30-day avg from snapshots; fall back to HV30.
+  const hist = getAverageAtmIv(symbol, 30);
+  let baselineIv = 0, baselineSrc: ScanResult["baselineSrc"] = "none", baselineN = 0;
+  if (hist && hist.avg > 0) { baselineIv = hist.avg; baselineSrc = "history"; baselineN = hist.n; }
+  else if (hv30 > 0)        { baselineIv = hv30;     baselineSrc = "hv30";    baselineN = 0; }
+
+  const ivJumpPct = baselineIv > 0 ? (atmIv - baselineIv) / baselineIv : 0;
+  const spike = ivJumpPct >= SPIKE_JUMP_PCT;
+  const bigMove = Math.abs(changePct) >= SPIKE_BIG_MOVE_PCT;
+
+  return {
+    symbol, spot, change, changePct, atmIv, hv30,
+    baselineIv, baselineSrc, baselineN,
+    ivJumpPct, ivHv, spike, bigMove, expiry,
+  };
 }