v3: Model-vs-Market scoreboard — frozen forecasts, fair scoring, live page

- prediction_snapshots: model + de-vigged market frozen ≤15min pre-kickoff
  (or at first sight of a live match, flagged 'late' and excluded from totals).
- src/lib/model/scoring.ts: ONE shared RPS/Brier/log-loss implementation for
  the backtest and the live scoreboard.
- /api/scoreboard + /scoreboard page: per-match model-vs-bookmaker bars, RPS
  for both after FT, 'closer' badges, running head-to-head with honest rules
  printed (small-sample caveat included). Nav: 'vs Market'.
- Methodology page now shows the three-way split + ensemble bake-off variants.
- Verified end-to-end with a simulated match: snapshot froze model 80.7% and
  DraftKings 67.0% (de-vig correct), both scored at FT, late-flag honored.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/src/db/db.ts

@@ -85,6 +85,14 @@ CREATE TABLE IF NOT EXISTS odds_history (
 );
 CREATE INDEX IF NOT EXISTS idx_odds_fixture ON odds_history(fixture_num, captured_at);
 
+CREATE TABLE IF NOT EXISTS prediction_snapshots (
+  fixture_num INTEGER PRIMARY KEY,
+  taken_at    INTEGER NOT NULL,
+  late        INTEGER NOT NULL DEFAULT 0,
+  model_json  TEXT NOT NULL,
+  market_json TEXT
+);
+
 CREATE TABLE IF NOT EXISTS ingest_log (
   id     INTEGER PRIMARY KEY AUTOINCREMENT,
   source TEXT NOT NULL,
@@ -229,6 +237,31 @@ export function latestOddsAll(): OddsRow[] {
     .all() as unknown as OddsRow[];
 }
 
+// ---- frozen pre-kickoff forecasts (Model-vs-Market scoreboard) ----
+export interface SnapshotRow {
+  fixture_num: number;
+  taken_at: number;
+  late: number;
+  model_json: string;
+  market_json: string | null;
+}
+
+export function hasSnapshot(fixtureNum: number): boolean {
+  return !!db().prepare('SELECT 1 FROM prediction_snapshots WHERE fixture_num = ?').get(fixtureNum);
+}
+
+export function saveSnapshot(fixtureNum: number, late: boolean, model: unknown, market: unknown | null): void {
+  db()
+    .prepare('INSERT OR IGNORE INTO prediction_snapshots (fixture_num, taken_at, late, model_json, market_json) VALUES (?, ?, ?, ?, ?)')
+    .run(fixtureNum, Date.now(), late ? 1 : 0, JSON.stringify(model), market ? JSON.stringify(market) : null);
+}
+
+export function allSnapshots(): SnapshotRow[] {
+  return db()
+    .prepare('SELECT fixture_num, taken_at, late, model_json, market_json FROM prediction_snapshots ORDER BY fixture_num')
+    .all() as unknown as SnapshotRow[];
+}
+
 // ---- per-fixture enrichment blob ----
 export function setMatchExt(fixtureNum: number, json: unknown): void {
   db().prepare('INSERT OR REPLACE INTO match_ext (fixture_num, json, updated_at) VALUES (?, ?, ?)')