refactor: extract DbDialect helper, inline ProfileValidationRules trait

Audit items #17 and #21.

#17 — DayOfWeekSignal and StickinessSignal each had their own
isSqlite ternary picking between SQLite (strftime/julianday) and
MySQL (DAYOFWEEK/DATEDIFF) date expressions. Centralised in
App\Services\Prediction\Signals\DbDialect.

#21 — ProfileValidationRules was a trait with one consumer
(CreateNewUser); inlined the rules into the action and deleted the
trait. Also dropped PasswordValidationRules::currentPasswordRules()
which was unused. PasswordValidationRules trait stays (two consumers).

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

app/Services/Prediction/Signals/DayOfWeekSignal.php

@@ -10,10 +10,7 @@ final class DayOfWeekSignal extends AbstractSignal
 
     public function compute(SignalContext $context): array
     {
-        $isSqlite = DB::connection()->getDriverName() === 'sqlite';
-        $dowExpr = $isSqlite
-            ? "(CAST(strftime('%w', price_effective_at) AS INTEGER) + 1)"
-            : 'DAYOFWEEK(price_effective_at)';
+        $dowExpr = DbDialect::dayOfWeekExpr('price_effective_at');
 
         $rows = DB::table('station_prices')
             ->where('fuel_type', $context->fuelType->value)

app/Services/Prediction/Signals/DbDialect.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace App\Services\Prediction\Signals;
+
+use Illuminate\Support\Facades\DB;
+
+/**
+ * SQL dialect helpers for the small set of MySQL/SQLite differences the
+ * signal classes care about. Centralises the isSqlite ternaries that were
+ * duplicated across DayOfWeekSignal and StickinessSignal.
+ */
+final class DbDialect
+{
+    private static function isSqlite(): bool
+    {
+        return DB::connection()->getDriverName() === 'sqlite';
+    }
+
+    /**
+     * Day-of-week expression returning 1=Sun..7=Sat (MySQL DAYOFWEEK convention).
+     * Targets a column on the queried table.
+     */
+    public static function dayOfWeekExpr(string $column): string
+    {
+        return self::isSqlite()
+            ? "(CAST(strftime('%w', {$column}) AS INTEGER) + 1)"
+            : "DAYOFWEEK({$column})";
+    }
+
+    /**
+     * Whole-day difference between MAX and MIN of a datetime column, suitable
+     * for use in an aggregate selectRaw.
+     */
+    public static function maxMinDayDiffExpr(string $column): string
+    {
+        return self::isSqlite()
+            ? "CAST((julianday(MAX({$column})) - julianday(MIN({$column}))) AS INTEGER)"
+            : "DATEDIFF(MAX({$column}), MIN({$column}))";
+    }
+}

app/Services/Prediction/Signals/StickinessSignal.php

@@ -8,10 +8,7 @@ final class StickinessSignal extends AbstractSignal
 {
     public function compute(SignalContext $context): array
     {
-        $isSqlite = DB::connection()->getDriverName() === 'sqlite';
-        $diffExpr = $isSqlite
-            ? 'CAST((julianday(MAX(price_effective_at)) - julianday(MIN(price_effective_at))) AS INTEGER)'
-            : 'DATEDIFF(MAX(price_effective_at), MIN(price_effective_at))';
+        $diffExpr = DbDialect::maxMinDayDiffExpr('price_effective_at');
 
         $rows = DB::table('station_prices')
             ->where('fuel_type', $context->fuelType->value)