refactor: persist EWMA only on LLM failure, dedup EWMA helper

Audit items #7 and #5.

#7 — BrentPricePredictor::generatePrediction previously wrote both an
EWMA row and an LLM row to price_predictions on every run. The
downstream OilSignal already prefers llm_with_context > llm > ewma, so
the EWMA row was dead weight 95% of the time. Now we try LLM first; if
it returns null (no API key, parse failure, etc.) we compute and persist
EWMA as a real fallback. This also avoids redundant work on the success
path.

Updated the "stores both" test to "stores only LLM" — asserts no EWMA
row is written when the provider succeeds.

#5 — BrentPricePredictor and AnthropicPredictionProvider both had
byte-identical computeEwma() methods with identical EWMA_ALPHA = 0.3
constants. Extracted to App\Services\Ewma::compute() and dropped both
private methods + their alpha constants.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/Services/Ewma.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace App\Services;
+
+/**
+ * Exponentially-weighted moving average. Pure function — used by
+ * BrentPricePredictor for the EWMA fallback prediction and by
+ * AnthropicPredictionProvider to enrich the basic-flow prompt.
+ */
+final class Ewma
+{
+    public const float DEFAULT_ALPHA = 0.3;
+
+    /** @param  float[]  $prices  Chronological order (oldest first). */
+    public static function compute(array $prices, float $alpha = self::DEFAULT_ALPHA): float
+    {
+        $ema = $prices[0];
+
+        foreach (array_slice($prices, 1) as $price) {
+            $ema = $alpha * $price + (1 - $alpha) * $ema;
+        }
+
+        return round($ema, 4);
+    }
+}