feat: Feature 2.1 — Parlay Scan with correlation detection + monetization

POST /api/scan/parlay — authenticated parlay analysis:
- Supabase JWT auth middleware (auth.getUser verification)
- 5 correlation types detected between legs (same_game, same_team,
  same_player_conflicting, positive_correlation, blowout_cascade)
- Overall parlay grading (A/B/C/D) with correlation penalty adjustments
- Free tier: 5 scans/month, atomic scan count increment
- Scan 5: full analysis + personalized upgrade pitch
- Scan 6+: 403 block with upgrade pitch
- Pitch personalization from scan history (top stats, grades, tier rec)
- DB writes: picks + scan_sessions per scan

30 new tests, 158 total (131 Node.js + 27 Python), all passing

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/unit/parlayGrader.test.js

@@ -0,0 +1,74 @@
+const { gradeParlayFromLegs } = require('../../src/services/parlayGrader');
+
+function makeLeg(grade, confidence, composite) {
+  return { grade, confidence, _composite: composite };
+}
+
+describe('parlayGrader', () => {
+  test('grade A: high composite, no D legs, no major_negative', () => {
+    const legs = [makeLeg('A', 85, 3.5), makeLeg('A', 90, 3.0), makeLeg('B', 72, 2.0)];
+    const result = gradeParlayFromLegs(legs, []);
+    expect(result.grade).toBe('A');
+    expect(result.confidence).toBeGreaterThanOrEqual(80);
+  });
+
+  test('grade B: moderate composite, at most 1 D leg', () => {
+    const legs = [makeLeg('B', 70, 2.5), makeLeg('B', 65, 2.0), makeLeg('D', 35, 0.3)];
+    const result = gradeParlayFromLegs(legs, []);
+    expect(result.grade).toBe('B');
+  });
+
+  test('grade C: low positive composite', () => {
+    const legs = [makeLeg('C', 55, 0.8), makeLeg('C', 50, 0.6)];
+    const result = gradeParlayFromLegs(legs, []);
+    expect(result.grade).toBe('C');
+  });
+
+  test('grade D: negative composite', () => {
+    const legs = [makeLeg('D', 35, 0.1), makeLeg('D', 30, -0.5)];
+    const result = gradeParlayFromLegs(legs, []);
+    expect(result.grade).toBe('D');
+  });
+
+  test('2+ D legs forces grade D', () => {
+    const legs = [makeLeg('A', 90, 4.0), makeLeg('D', 35, 0.2), makeLeg('D', 30, 0.1)];
+    const result = gradeParlayFromLegs(legs, []);
+    expect(result.grade).toBe('D');
+  });
+
+  test('major_negative caps grade at B', () => {
+    const legs = [makeLeg('A', 90, 4.0), makeLeg('A', 85, 3.5)];
+    const flags = [{ type: 'same_player_conflicting', legs: [0, 1], impact: 'major_negative' }];
+    const result = gradeParlayFromLegs(legs, flags);
+    expect(['B', 'C', 'D']).toContain(result.grade);
+  });
+
+  test('minor_negative reduces composite by 0.3 each', () => {
+    const legs = [makeLeg('B', 70, 2.0), makeLeg('B', 68, 1.8)];
+    const withoutFlags = gradeParlayFromLegs(legs, []);
+    const withFlags = gradeParlayFromLegs(legs, [
+      { type: 'same_game_same_team', legs: [0, 1], impact: 'minor_negative' },
+    ]);
+    expect(withFlags.composite).toBeCloseTo(withoutFlags.composite - 0.3, 1);
+  });
+
+  test('confidence adjusted: -5 per minor, -15 per major', () => {
+    const legs = [makeLeg('B', 80, 2.0), makeLeg('B', 80, 2.0)];
+    const noFlags = gradeParlayFromLegs(legs, []);
+    const minorFlag = gradeParlayFromLegs(legs, [
+      { type: 'test', impact: 'minor_negative' },
+    ]);
+    const majorFlag = gradeParlayFromLegs(legs, [
+      { type: 'test', impact: 'major_negative' },
+    ]);
+    expect(minorFlag.confidence).toBe(noFlags.confidence - 5);
+    expect(majorFlag.confidence).toBe(noFlags.confidence - 15);
+  });
+
+  test('confidence clamped to 30-95', () => {
+    const legs = [makeLeg('D', 35, 0.1)];
+    const manyFlags = Array(5).fill({ type: 'test', impact: 'major_negative' });
+    const result = gradeParlayFromLegs(legs, manyFlags);
+    expect(result.confidence).toBe(30);
+  });
+});