feat: Feature 1.3 — Prop Analysis Engine with 6-step grading pipeline

Core intelligence for BetonBLK prop analysis:
- POST /api/analyze/prop — single prop analysis
- POST /api/analyze/batch — multi-prop analysis for parlay scanner
- 6-step pipeline: season avg → recent form → situational splits →
  cross-book lines → kill conditions → grade (A/B/C/D)
- 6 kill conditions: low_minutes, small_sample, b2b_high_usage,
  blowout_risk, split_conflict, no_opponent_data
- Composite scoring with confidence (30-95), bonuses, penalties
- Added spreads market to Odds API fetch (zero extra credits)
- Full reasoning output with step-by-step breakdown

36 new tests (unit + integration), 128 total across all features

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

src/utils/signals.js

@@ -0,0 +1,15 @@
+function deltaToSignal(delta) {
+  const abs = Math.abs(delta);
+  if (abs < 0.5) return delta >= 0 ? 'neutral' : 'neutral';
+  if (abs < 2.0) return delta >= 0 ? 'lean' : 'lean_bearish';
+  if (abs < 4.0) return delta >= 0 ? 'bullish' : 'bearish';
+  return delta >= 0 ? 'strong_bullish' : 'strong_bearish';
+}
+
+function directedDelta(avg, line, direction) {
+  // For "over", positive delta is good. For "under", negative delta is good.
+  const raw = avg - line;
+  return direction === 'under' ? -raw : raw;
+}
+
+module.exports = { deltaToSignal, directedDelta };