Recency-weighted v4 model: fast-Elo blend, Team-DC majority, nightly refit

Validated on the strict 2018-2022 window and confirmed on the untouched
2022-2026 test set (RPS 0.1703 vs 0.1721 over 4,448 matches):
- the Elo member now blends 30% of a 3x-faster Elo walk, so recent
  results move ratings much harder
- ensemble weight shifts from 75/25 toward Elo to 45/55 toward the
  time-decayed Team-DC member — the recent-form model now leads
- Team-DC refits nightly (and at boot) on the 15-year window plus every
  finished 2026 match, via a new committed-at-build dcTrain.json
  (server-only, excluded from the PWA precache)
- a recent-form goal multiplier was also tested and did NOT validate;
  it ships disabled, and backtest.json records the whole decision

buildBacktest.ts grew the experiment harness: wider xi/k grids, the
fast-Elo and form variants, a pre-registered ship bar (>=0.0005
validation RPS), and a 7-day refit-cadence check. Older ratings.json
files still work — all new model fields are optional with golden
regression coverage.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/refit.ts

@@ -0,0 +1,51 @@
+// Pure helpers for the nightly in-tournament Team-DC refit (no fs/db imports,
+// so vitest can load them). The training base comes from dcTrain.json — compact
+// [date, home, away, hs, as, neutral01] rows with absolute dates — plus every
+// finished World Cup fixture so far.
+import { hostAdvantage } from '../../src/lib/model/hosts';
+import type { DcMatch } from '../../src/lib/model/teamDc';
+import type { Fixture } from '../../src/lib/types';
+
+export type DcRow = [string, string, string, number, number, number];
+
+const DAY = 24 * 60 * 60 * 1000;
+
+/** Build the refit training set, with daysAgo relative to the newest match in
+ *  the combined data. Host-nation fixtures are oriented so the host is the
+ *  DC home side (the CSV convention for non-neutral rows). */
+export function refitTrainingSet(
+  base: { asOf: string; rows: DcRow[] },
+  finished: Fixture[],
+  homeAdvElo: number,
+  windowYears = 15,
+): { train: DcMatch[]; asOf: string } {
+  let asOf = base.asOf;
+  const extra: { date: string; home: string; away: string; hs: number; as: number; neutral: boolean }[] = [];
+  for (const f of finished) {
+    if (!f.home.team || !f.away.team || f.homeScore == null || f.awayScore == null) continue;
+    const date = f.kickoff.slice(0, 10);
+    if (date > asOf) asOf = date;
+    const adv = hostAdvantage(f.home.team, f.away.team, f.venue, homeAdvElo);
+    if (adv < 0) {
+      extra.push({ date, home: f.away.team, away: f.home.team, hs: f.awayScore, as: f.homeScore, neutral: false });
+    } else {
+      extra.push({ date, home: f.home.team, away: f.away.team, hs: f.homeScore, as: f.awayScore, neutral: adv === 0 });
+    }
+  }
+
+  const nowT = new Date(asOf).getTime();
+  const windowMs = windowYears * 365 * DAY;
+  const train: DcMatch[] = [];
+  for (const [date, home, away, hs, as, neutral] of base.rows) {
+    const t = new Date(date).getTime();
+    if (nowT - t > windowMs) continue;
+    train.push({ daysAgo: (nowT - t) / DAY, home, away, homeGoals: hs, awayGoals: as, neutral: neutral === 1 });
+  }
+  for (const m of extra) {
+    train.push({
+      daysAgo: (nowT - new Date(m.date).getTime()) / DAY,
+      home: m.home, away: m.away, homeGoals: m.hs, awayGoals: m.as, neutral: m.neutral,
+    });
+  }
+  return { train, asOf };
+}